Chemistry of Iron Sulfides - American Chemical Society

Chem. Rev. 2007, 107, 514−562

514

Chemistry of Iron Sulfides David Rickard*,† and George W. Luther III‡ 1School

of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF103YE, Wales, U.K., and College of Marine and Earth Studies, University of Delaware, Lewes, Delaware 19958 Received June 22, 2006

Contents 1. Introduction 1.1. Iron Sulfides in Marine Systems: Background 2. Sulfur Chemistry 2.1. Sulfide Chemistry in Aqueous Solutions 2.2. Polysulfide Chemistry 3. Iron Chemistry 4. Iron Sulfide Complexes and Clusters 5. Iron(II) Monosulfide: Mackinawite 5.1. Mackinawite Structure 5.2. Mackinawite Composition 5.3. Mackinawite Solubility 5.6. Kinetics and Mechanism of Mackinawite Formation in Aqueous Solutions 5.7. Kinetics and Mechanism of Mackinawite Dissolution in Aqueous Solutions 6. Iron Thiospinel, Greigite 6.1. Greigite Structure 6.2. Greigite Composition 6.3. Greigite Solubility 6.4. Reactions To Form Greigite 7. Iron Disulfide, Pyrite 7.1. Pyrite Structure 7.2. Pyrite Composition 7.3. Pyrite Solubility 7.4. Pyrite Formation 7.4.1. Mackinawite, FeSm, as a Reactant 7.4.2. Greigite, Fe3S4g, as a Reactant 7.4.3. Pyrite Nucleation and Crystal Growth 7.4.4. Polysulfide-Controlled Pyrite Reaction Kinetics: The Bunsen Reaction 7.4.5. Sulfide Controlled Reaction Kinetics: The Berzelius Reaction 7.4.6. Relative Rates of Pyrite Formation 7.4.7. Kinetics of Pyrite Crystal Growth 8. Other Iron Sulfides 8.1. Cubic FeSc 8.2. Troilite, FeSt, and the Pyrrhotite Group 8.3. Smythite, Rhombohedral Fe9S11s 8.4. Marcasite, Orthorhombic FeS2m 8.5. Other Iron Sulfides 9. The FeS System in Biochemistry and for Organic Compound Formation

514 515 518 518 520 523 530 533 533 534 535 537 538 538 539 540 540 541 543 544 544 544 546 549 550 550 551 552 552 554 554 554 555 555 555 555 555

* To whom correspondence should be addressed. Phone: +44(0)2920874284. Fax: +44(0)2920874326. E-mail: [email protected]. † Cardiff University. ‡ University of Delaware.

9.1. Pyrite Formation Proposal 9.2. FeS Membrane Proposal 10. Acknowledgments 11. References

556 558 558 558

1. Introduction The iron sulfides constitute a diverse group of solids and dissolved complexes many of which play key roles in marine systems. The group is particularly complex in the lower temperature environments characterizing much of the Earth’s surface where a number of metastable phases become significant in biogeochemical processes. At least seven different solids consisting only of iron and sulfur are known to occur naturally at these temperatures (Table 1). At higher temperatures, the chemistry appears to be simpler although this may be partly a consequence of the experimental difficulties of probing supercritical and often salt-rich aqueous fluids. Certainly the number of iron sulfide products, as seen in the geological record, appears more restricted. This review is concerned with the complex lower temperature chemistry of iron sulfides, which is more directly related to marine systems. There is an interface between this lower temperature regime and the higher temperature regime in the deep ocean hydrothermal vents, where iron sulfide fluids debouch into the deep oceans at temperatures up to 400 °C. We dip our feet tentatively into these waters at the end of this review. Iron sulfides are an intrinsic and essential part of the global biogeochemical sulfur cycle. They provide evidence for the processes and fluxes occurring in the sulfur cycle. Indeed, because of the relationship between the sulfur cycle and the other key cycles of carbon and oxygen, this evidence is an essential part of what we understand about how the Earth works. The evolution of the global biogeochemical sulfur cycle is a major aspect of the evolution of the Earth because the sulfur cycle is intimately involved in the cycles of a number of key elements including oxygen and carbon, and thus life itself. Because of their role in the iron cycle, iron sulfides also provide information about the biogeochemistry of metals. Iron sulfides are also key indicators of contemporary environmental conditions. The consequence of this is that iron sulfides are central to our understanding of the evolution of the Earth. Many current fundamental concepts about the evolution of the Earth surface environment (e.g., the evolution of atmospheric O2) are mainly based on analyses and interpretations of iron sulfides, particularly pyrite, in ancient sedimentary rocks. As the rocks get older, sedimentary iron sulfides become progressively more central to contemporary debates about Earth history. Indeed, they

10.1021/cr0503658 CCC: $65.00 © 2007 American Chemical Society Published on Web 01/30/2007

Chemistry of Iron Sulfides

David Rickard trained in geology, chemistry, and microbiology at Imperial College, London, where he obtained his B.Sc. (1965) with first class honours and Ph.D. (1968). He was awarded an ARSM in 1965, DIC in 1968, Docentur at Stockholm University in 1975, and C.Eng in 1978. He was awarded Honorary Fellowships of the Geochemical Society, the European Association of Geochemistry, and the Society of Geology Applied to Mineral Deposits in 2003. He was Chair of the Department of Earth Sciences at Cardiff University (1983−2002) having previously worked in Sweden and Australia, and currently holds the Chair in Geochemistry. He has published over 100 scientific papers and books on sulfide chemistry, geochemistry, biogeochemistry, and ore geology. He was an Editor of Economic Geology (1975−1980), was an Editor of Mineralium Deposita (1982−2000), and has been a Chief Editor of Chemical Geology since 2004.

have been associated as catalysts in a number of key biochemical reactions associated with the “iron-sulfur world” hypothesis for the development of life. This is the third research review on metal sulfides, especially iron sulfides, that the senior author has published over the last 2 years. The first review1 examined the nature and distribution of HCl-soluble iron sulfides in sediments. The second review2 summarized current knowledge on the stability, structure, and composition of metal sulfide complexes and clusters in low-temperature aqueous systems. This research review examines Fe and S chemistry in the context of iron sulfide formation processes and their kinetics with particular reference to marine systems. The emphasis is on the chemistry that occurs or can occur within the natural environment but does not emphasize in great detail the distributions of relevant chemical species in the environment. Readers are referred to the other reviews for updates on these aspects of sulfur chemistry in natural systems. In this review, we have tried to join up the dots of hundreds of recent papers to paint a consistent picture of current knowledge about the chemistry of iron sulfides. However, as with the two previous reviews, it is not merely a compilation of previous data but also presents new information based on (mainly) published observations and measurements. The research themes developed include the following: • A re-evaluation of the equilibrium thermodynamics of the aqueous iron, sulfide, and iron sulfide systems. • The importance of carefully distinguishing between kinetics and equilibrium thermodynamics in understanding iron sulfide chemistry. • The consistency of apparently conflicting reports on iron sulfide chemistry in terms of the equilibrium and kinetics approaches. This review does not discuss the application of the chemical results to understanding the formation of the extraordinary variety of textures and habits exhibited by

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George W. Luther III was born in Philadelphia, Pennsylvania, in 1947. He attended La Salle College where he received an ACS certified B.A. degree in 1968. He received a National Defense Education Act Fellowship from the University of Pittsburgh, where he graduated in 1972 with a Ph.D. in physical−inorganic chemistry. His studies included NMR spectroscopy and boron hydride chemistry with Dr. James C. Carter. Upon graduating from the University of Pittsburgh, he became an assistant professor at Kean College of New Jersey, where he rose through the ranks and became department chair (1976−1984) and helped the program become ACS certified. During this time, he also was active in ACS local section activities and became chair of the New Jersey ACS local section in 1983. In 1986, he moved to the University of Delaware, where he began a research program in marine inorganic chemistry, including studies on trace metal, sulfur, and iodine speciation in environmental waters. His work emphasizes the use of physical chemistry to understand environmental problems. Specific examples include the use of chemical kinetics and frontier molecular orbital approaches to understand redox and metal complexation reactions. In the early 1990s, he began developing in situ voltammetric sensors to determine chemical species in sedimentary pore waters and the water column. He has sailed on many cruises studying the chemistry of the major oceans, the hydrothermal vents, the Black Sea, the Arabian Sea, and the Mediterranean Sea. In 1997, he was elected chair of the ACS Division of Geochemistry and in 2004 received the Claire C. Patterson award from the Geochemical Society.

pyrite in marine systems. This is a rapidly developing field, which would constitute an equally large review paper. The review focuses on the work on the chemistry of iron sulfides since 1987, when Morse et al.3 published their benchmark review. Readers interested in the earlier history of the subject are directed to that study, although there are some excursions into the past in the present review as these are required for completion or to explain where some modern ideas originated.

1.1. Iron Sulfides in Marine Systems: Background The biogeochemical sulfur cycle is not well-defined. Pre1979 and before the discovery of deep sea hydrothermal vents,4 it was thought that sulfide in the surface environments of the Earth is almost entirely a result of the microbial reduction of sulfate. As discussed below, abiologic sulfate reduction at temperatures below ∼150 °C is kinetically extremely slow, even over geologically significant time periods. By contrast, microbial respiration with sulfate is abundant because sulfate is far more soluble in water than dioxygen and therefore it is a more abundant electron acceptor for microbial respiration. Earlier understanding of the biochemical sulfur cycle included a major imbalance in the marine fluxes, which appears to have been largely explained by the influence of deep sea hydrothermal vent activity.5 However, a number of other processes, including the discoveries of the sulfate-reduction-driven deep bio-

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Table 1. The Solid Phases in the Fe-S System material

composition

structure

mackinawite

FeSm

tetragonal P4/nmm

cubic FeS

FeSc

cubic F4h3m

troilite

FeSt

hexagonal P6h2c

pyrrhotite

Fe1-xS

monoclinic, for example, A2/a; hexagonal P6/mmc

smythite

Fe9S11s

hexagonal R3m

metastable phase related to the Fe1-xS group

greigite

Fe3S4g

cubic Fd3m

metastable FeIIFeIII sulfide; the thiospinel of iron

pyrite

FeS2p

cubic Pa3

stable iron(II) disulfide known as “fool’s gold”

marcasite

FeS2m

orthorhombic Pnnm

metastable iron(II) disulfide

sphere6 and the sulfate-reduction-driven anaerobic oxidation of methane7 and a re-appraisal of the significance of organic sulfur burial,8,9 may also contribute significantly to the global sulfur budget. The average oceanic sulfate concentration is 0.02824 mol kg-1 10 and, with an oceanic mass of 1.4 × 1024 g, the mass of sulfate in the oceans is currently 1.44 × 1021 g. The total rate of sulfate reduction is 3 × 1014 g of S a-1 11 of which 75-90% is recycled to produce sulfate and intermediate valence sulfur species.12 The rate of iron sulfide formation (as pyrite) is currently, therefore, at least 5.6 × 1012 g a-1. That is, about 5 million tons of pyrite are produced every year in the oceans. Respiration via microbial sulfate reduction is the most important pathway for mineralization of organic matter in marine sediments.9,13-16 Indeed, sulfate also drives anaerobic methane oxidation in sediments.7 Since the mineralization and burial of organic matter ultimately determines the oxygen content of the atmosphere, the fixation of pyrite sulfur in sediments is largely responsible for the oxygenated surface environment of the planet at the present and through much of geological time.17 Despite the present day oxidized ocean, Fe(II)- and S(-II)-rich environments can be found in sedimentary pore waters and in deep waters of anoxic basins. Permanently anoxic basins (defined operationally as environments with O2(aq) < 10-6 M18 but also containing tens of micromolar to several millimolar S(-II)) are well represented by the Black Sea,19 the Cariaco Trench,20 and fjords such as the Framvaren Fjord in Norway.21 Seasonally anoxic basins are ubiquitous on all the continents and the upper Chesapeake Bay22 is a classic example. In these basins, Fe(II) and S(-II) are present during the summer months in the deep water. In all these examples, surface waters contain O2.

properties metastable material that is the major constituent of the FeS precipitated from aqueous solutions highly unstable phase formed before FeSm stoichiometric end member of the Fe1-xS group nonstoichiometric stable group where x > 0.2; monoclinic form is approximately Fe7S8; hexagonal form is approximately Fe10S11

natural abundance widespread mineral in low-temperature aqueous environments not found naturally mainly found in meteorites most abundant iron sulfides in the Earth and solar system; rare in marine systems rare mineral mainly found in hydrothermal systems usually associated with carbonates fairly widespread mineral particularly associated with fresh water systems the most abundant mineral on the Earth’s surface locally common mineral in hydrothermal systems and in sedimentary rocks

It is important not to confuse the chemical terms oxic, suboxic, and anoxic with the biological terms aerobic, dysaerobic, and anaerobic. The chemical terms refer to the concentration of molecular O2 present in the system, whereas the biological terms refer to the characteristics of the organisms inhabiting specific environments. Although there is an obvious overlap, there are significant differences. Thus the chemical terms are operational and based on the limitations of equipment available at the time they were defined. From the point of view of the iron sulfides, for example, 10-6 M O2 can be a significant amount of dioxygen, especially in open systems. Likewise, many organisms are not obligate in their oxygen sensitivity but are facultative and able to adapt to changing oxygen concentrations. In the absence of physical forcing, a redox sequence13 based on the thermodynamics of the decomposition of organic matter by microorganisms using natural oxidants (O2, NO3-, MnO2, FeOOH, SO42-) is produced and gives welldefined vertical profiles for O2, NO3-, Mn(II), Fe(II), H2S, and SO42- as shown in Figure 1. The idealized profiles given are dependent on the input of inorganic and organic matter debris from surface waters to bottom waters and sediments via sedimentation. Organic matter brings with it the hard parts of organisms, which include SiO2 and CaCO3, as well as other elements that are important to life processes such as Fe and Mn. The smooth profiles in Figure 1 are governed by diffusion of the soluble species and can be found in sedimentary pore waters13 and the water column of enclosed anoxic basins.19 The nonoverlap of O2 with Mn2+ is based on voltammetric microelectrode data.23 Where soluble species overlap or cross oVer as in the Fe and S cycles, redox or precipitation/dissolution reactions or both occur to remove these species from solution. In the ideal case, the gradients of two chemical species that cross over should balance



Figure 2. Iron catalytic cycle with O2 and H2S at oxic-anoxic interfaces.

In the case of the reaction of O2 with S(-II), the first electron-transfer step is thermodynamically unfavorable (eq 1)

HS- + O2 f SH + O2Figure 1. Schematic representation of trends in pore water profiles with depth below the sediment-water interface and concentration axes in arbitrary units.

stoichiometrically. If they do not, then other reactions or processes need to be considered. Figure 1 demonstrates that O2 is depleted before the reduced forms of Fe and S are measurable. The zone between the last detection of O2 and the first detection of S(-II) is called suboxic. The definition is again operational: the limits of detection of dissolved O2 by conventional methods are >10-6 M and, of course, much greater in the presence of S(-II). In near-shore sedimentary environments, O2 may be depleted in 1 or 2 mm,24 whereas in the deep ocean, O2 will be depleted after several centimeters.25,26 In sediments, the thickness of the suboxic zone can be on the order of a few millimeters to centimeters27 or several meters.19 Anoxic basins show a suboxic zone thickness from centimeters (e.g., Framvaren Fjord21,28) to a meter (e.g., Bannock Basin29) to tens of meters (e.g., Black Sea19 and Cariaco Basin30). The smooth profiles shown in Figure 1 are disrupted by physical forcing30-33 and animal/plant activity (bioturbation).34,35 For suboxic water columns, vertical and lateral mixing of water masses in ocean basins, storms, and seasonal weather patterns affect the idealized profile by forcing O2 deeper into the water column. For sedimentary pore waters, storms and bioturbation mix the upper most sediments. Wave action from storms pounds shallow sediments along the coast and brings O2 and NO3- into the reducing sediments. The growth of plants in sediments and the burrowing of organisms into sediments (bioturbation) also mixes oxidized and reduced zones. In addition to dissolved O2 and NO3- being forced deeper into reducing environments to oxidize reduced materials, solid-phase iron(III) and manganese(III,IV) (oxy)hydroxides and oxides are mixed from surface sediments to reduced sediments as sulfide minerals are moved to the surface. These processes disrupt the smooth profiles and allow for significant redox chemistry to occur, for example, the oxidation of sulfide by Fe(III) and Mn(III,IV) phases.

E° ) -1.24 V

(1)

and this leads to both the Mn and Fe cycles acting as catalysts to oxidize S(-II). The Mn36 and Fe37 catalytic cycles are composed of (1) the reaction of O2 with Mn(II) and Fe(II) to produce Mn(III,IV) and Fe(III) chemical species and (2) the oxidation of S(-II) by the Mn(III,IV) and Fe(III) species, which leads to the re-formation of Mn(II) and Fe(II) and the creation of electrophilic and nucleophilic sulfur compounds with intermediate oxidation states such as S8, Sn(-II), and S2O3(-II) (Figure 2). Another consequence of one electron-transfer reactions is the possibility of forming reactive oxygen species (ROS) such as O2- and H2O2 at the oxic/anoxic transition zone. Also shown in Figure 1 is the reaction of Fe(II) with MnO2,38 which can produce Fe(III) compounds and thus transfer oxidation character deeper into the reducing zone. Once Fe2+ forms, it reacts with Sn(-II) and S(-II) to form FeS and FeS2 (mainly as pyrite). In environments that experience seasonal oxic conditions, the formation of Fe(III) species or mixed Fe(II,III) solids (termed “reactive Fe”) occurs and can be greater than the formation of FeS2.39-41 The number of reactive iron solid phases that can form is considerable and their rates of reactivity with sulfide, other reductants, and microbes can vary over several orders of magnitude.42-44 For example, Fe(III) in silicates is more resistant to reduction and dissolution by sulfide than Fe(III) crystalline materials such as goethite (FeOOH), hematite (Fe2O3), and magnetite (Fe3O4), which are less reactive than ferrihydrite (more-or-less Fe(OH)3, see discussion below) and other nanocrystalline materials. The reaction of these solid reactive iron phases with sulfide results in a steady increase of FeS2 (at the expense of any intermediate FeS phases) with depth in the sediments. In the anoxic zone where sulfate decreases toward nondetectible levels, essentially all reactive Fe (and Mn(III,IV)) phases are reduced and the formation of FeS2 reaches a maximum. Once all the Fe is converted to FeS2, H2S concentrations build up and reach a maximum as sulfate no longer readily diffuses to deeper depths. Where


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Figure 3. The sulfur cycle in the environment. Dashed lines indicate the reaction of inorganic sulfur compounds to form organosulfur compounds. Question marks indicate that the reactant has not been verified for a given sulfur species transformation.

sulfate approaches nondetectible levels, methanogenesis occurs with overlap of sulfate and methane profiles indicating that sulfate is the oxidant for anaerobic methane oxidation.45 Although sulfate reduction in the deep anoxic zone is limited by the depleted sulfate and metabolizable organic matter contents, sulfate-reducing prokaryotes exist in the deep subsurface and are active.6 The burial of FeS2 in sediments has received significant attention (e.g., ref 46). For normal marine sediments with overlying water containing dissolved O2, formation and burial of FeS2 occurs in the sediments and is termed diagenetic (formation after reactive Fe deposition). For sediments that have H2S in the overlying waters, reactive Fe compounds, which sink to the bottom sediments, react with the sulfide and form FeS2 in the water column. This pathway is termed syngenetic and has been documented for ancient euxinic sediments, which had no O2 in their overlying waters. The latter pathway also indicates that the sediments are Felimited; that is, there is no fresh input of reactive Fe solid phases for diagenetic FeS2 formation. The Black Sea, a modern nearly euxinic system, is an example where FeS2 forms by both pathways. Figure 3 is a schematic diagram of the sulfur cycle in the environment. Although the Fe-S system can be dominated by FeS2 burial in sediments, organic sulfur burial can equal or exceed FeS2 burial in some saltmarsh and near-shore environments.47-49 The range and complexity of sulfur compounds and oxidation states are evident in Figure 3 including two pathways for organic sulfur oxidation, as well as the reaction of inorganic sulfur compounds with organic compounds and CO/CO2 to form organosulfur compounds. In this review, we center our efforts primarily on the inorganic chemistry of the sulfur cycle.

2. Sulfur Chemistry 2.1. Sulfide Chemistry in Aqueous Solutions Free sulfide, S(-II), exists in aqueous solutions mainly in the form of H2S and HS- with minor S2-. HS- is a Lewis

Figure 4. Molecular orbital energy diagrams for HS- and H2S. Modified from Rickard and Luther2 and reproduced by permission of the Mineralogical Society of America, Copyright 2006.

base whereas H2S can act as a Lewis base or acid. The frontier orbitals for the molecule H2S (S-H-S bond angle 92°) are well-known.50 Figure 4 shows the molecular orbital energy diagram for H2S, which results from the linear combination of the 1s orbitals of the two hydrogen atoms and 3s and 3p orbitals of the sulfur atom. It also compares the energy level diagrams of HS- with H2S. The energies of these orbitals are an important feature of their reactivity. The lowest unoccupied molecular orbital (LUMO) for HSwas calculated to be +8.015 eV51 with no experimental data available for comparison. However, the high positive energy indicates HS- cannot be an electron acceptor. The highest occupied molecular orbital (HOMO) for HS- was calculated to be -2.37 eV, which compares well with the experimental value of -2.31 eV.52,53 The HOMO for HS- is less stable than that for H2S (-10.47 eV) indicating that HS- is more nucleophilic and basic than H2S, consistent with known reactivity. The HOMO orbital for H2S was calculated to be



-9.646 eV, whereas the experimental value from ionization energy data is -10.47 eV.52,53 Thus, H2S is not an excellent electron donor because the HOMO is so stable. H2S can act as an electron donor to Fe(II) because Fe(II) has LUMO orbitals with more stable energies compared with the HOMO of H2S or has an empty orbital due to water exchange. The LUMO orbital for H2S using the semiempirical approach was calculated to be +0.509 eV51 whereas the experimental value is -1.1 eV based on electron affinity data.53 These values bracket the LUMO for O2 (-0.47 eV), for example, and suggest that H2S can be an excellent electron acceptor.51,54 The LUMO orbital for H2S (termed 3a1) is made from the combination of 1s orbitals from each hydrogen and the px orbital of sulfur, which also mixes with the s orbital of sulfur.50 The molecular orbital is delocalized across all three atoms since the sign of the wave function encompasses all atomic centers. Figure 5 shows the molecular orbital and the charges from ab initio calculations.55 Because the LUMO is an antibonding orbital in the bent H2S molecule, it is more destabilized relative to similar molecular orbitals for linear molecules such as BeH2.50 Because of this destabilization, electrons added to this LUMO orbital cause a weakening of both S-H bonds. The chemistry of S(-II) in aqueous environmental systems has been fairly well constrained.3 At infinite dilution, pK1(H2S) is 6.98 ( 0.03 at 25 °C and 1 bar,56 which means that H2S dominates the system at acid pH values and HS- is the dominant species in alkaline solutions. In seawater between 5 and 25 °C and salinities of 5 to 40, the value of pK1(H2S) in seawater, pK1*(H2S), can be described as a function of both temperature, T (in kelvin), and salinity, S.57

Figure 5. LUMO for H2S. Upper panel is a three-dimensional representation; lower panel is a two-dimensional representation with charges from Trsic and Laidlaw.55 The positive sign indicates the positive part of the orbital’s wavefunction and the negative sign the negative part of the orbital’s wavefunction.

pK1* ) pK1 + AS1/2 + BS where

pK1 ) -98.080 + 5765.4/T + 15.0455 ln T A ) -0.1498 B ) 0.0119 These data suggest that in standard seawater (S ) 35) at 25 °C, pK1(H2S) is 6.51 ( 0.03, which is 0.47 logarithmic units less than the value at infinite dilution. At 5 °C, pK1(H2S) is 7.33 ( 0.03, which compares with 6.86 ( 0.03 in standard seawater. The pK2(H2S) is less precisely constrained but is estimated to be >18.58,59 The uncertainty stems from the problems of polysulfide contamination in the experimental measurement of pK2(H2S). Although it has been reported that the high value of pK2 means that S2- does not have significant concentration in aqueous solutions, this is not actually the case. As shown in Figure 6, the activity of the S2- ion in equilibrium with a total S(-II) activity of 10-3 varies between approximately 10-12 at pH ) 10 and 10-20 at pH ) 4. In thermodynamic terms, these are significant activities compared with the solubility of many metal sulfide minerals (see ref 2 for example). The reason why S2- is not considered in low-temperature aqueous sulfide chemistry is not due to its low concentration but due to the uncertainty in pK2(H2S), which propagates through equilibrium systems. Thus the solubility of metal

Figure 6. Distribution of aqueous sulfide species at {S(-II)}T ) 10-3 in terms of pH.

sulfides is usually written in terms of the HS- species, Ksp,2 for reactions like

MS + H+ ) M2+ + HS-

(2)

rather than in terms of the S2- species, Ksp,

MS ) M2+ + S2-

(3)

The problem is that some compilations of stability constants still include older pK2(H2S) values around 12 or 14 or include sulfide solubility constants that are based on these older values. These still slip readily into the literature since thermodynamic databases may include these intrinsic errors as pointed out originally by Schoonen and Barnes.59 Microbial sulfate reduction is carried out by a number of prokaryotes, including bacteria and archaea, and we refer to these collectively as sulfate-reducing prokaryotes (SRPs) and the process as prokaryotic sulfate reduction (PSR). The process, which involves an eight-electron reduction, proceeds through a number of stages. SO42- is attached to the phosphate of ATP forming adenosine phosphosulfate (APS) catalyzed by ATP sulfurylase. The APS SO4(-II) moiety is


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reduced to SO3(-II) with the release of AMP. The subsequent reduction of the SO3(-II) moiety is relatively facile. The process involves a considerable fractionation of the S isotopes, conventionally measured in terms of the 34S/32S ratio. The measurements are present in terms of the per thousand deviation of the 34S/32S ratio from that of a meteoritic standard, δ34S ‰. Fractionations of the order of -30‰ are common in PSR and are used as proxies for identifying PSR in ancient and modern environments. The main part of the fractionation takes place at the initial reduction of APS. The abiologic reduction of sulfate is kinetically extremely slow at low temperatures (e.g., less than ca. 150 °C) except in the most extreme conditions with powerful reductants and relatively high concentrations, such as HI and acidified Cr2+. The reason is the high symmetry of the sulfate ion, and this is demonstrated by the observation that SO32- is readily reduced. In contrast the oxidation of S(-II) to SO4-2 is kinetically more amenable. The result is that the interchange

SO42- T S(-II)

(4)

is abiologically irreversible in low-temperature natural environments. The predominance of PSR in sulfide formation means that in reality the reduction of SO42- to S(-II) in natural systems is kinetically fast and equilibrium may be approached. It is also important for theoretical equilibrium computations involving sulfate and sulfide: in the absence of microorganisms, the sulfate-sulfide couple is not reversible and the equilibrium approximation is invalid. A Pourbaix60 or pH-Eh diagram of the stable phases in the S- H2O system at 25 °C and 1 bar total pressure is shown in Figure 7. This diagram has several interesting features. The boundary between the dominance fields of SO42- and S(-II) is widely assumed to be co-incident with the boundary between oxic and anoxic sulfide-bearing systems. In fact, of course, this is not actually true as is shown in Figure 1. The boundary is merely a convenient way of expressing redox in the S-H2O system. It denotes the locus of points where {SO42-} ) {S(-II)}. Sulfide occurs on both sides of the boundary. At pH ) 8, for example, the relationship between {SO42-} and {S(-II)} is given by the reaction

HS- + 4H2O ) SO42- + 9H+ + 8e-

(5)

log {SO42-}/{HS-} ) (135.14)Eh - 33.52

(6)

where

The {SO42-}/{HS-} boundary is then at Eh ) -0.247 V. But at {S}T ) 10-3, {S(-II)} is still present at 10-6, a significant activity, at Eh ) +0.022 V. Likewise, SO42- is still present at significant activities deep into the sulfide zone. The latter is, of course, important for SRPs, which require a supply of SO42-. Mass-dependent S isotope fractionation as it variously impinges on the sedimentary sulfur system has recently been widely reviewed.9,61,62 Much of the information regarding the sulfur isotope systematics in natural systems is based on the sulfur isotopic composition of pyrite. Earlier work was limited by the analytical considerations, in particular, the need to collect sulfur from bulk sediments in order to obtain enough sulfur for the isotopic composition to be measured precisely. Bulk analyses give the average isotopic composition of the pyrite. Technical advances have enabled the sulfur

Figure 7. pH-Eh diagram for stable sulfur species in aqueous solution (25 °C, 1 bar total pressure, ∑{S} ) 10-3).

isotopic composition of single pyrite grains to be analyzed, and these measurements have revealed considerable variations in isotopic compositions between individual sedimentary pyrite grains and even within single pyrite grains.63 The variation in the sulfur isotopic compositions between and within pyrite grains can encompass a large fraction of the total variation in bulk sulfur isotope in the sediment. Intragrain analyses show that sulfur isotopes normally become heavier toward the outside of the pyrite grain. The degree of S isotope fractionation is related to the concentration of SO42-, which limits the rate of PSR at 9. The


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Table 3. Stability Data for Polysulfide Ions and Their Conjugate Acids for the Dissociation Reactions Written in Terms of HS-, SO42-, and H+ of the Type Used in the Database of Equilibrium Calculation Programs Like GWBa Sn-

reaction

2 3 4 5 6 7 8

∆G°f(Sn2-)

∆G°r(Sn2-)

log K(Sn2-)

S2 + H2O ) 1.75HS + 0.25SO4 + 0.25H S32- + 2H2O ) 2.50HS- + 0.50SO42- + 1.50H+ S42- + 3H2O ) 3.25HS- + 0.75SO42- + 2.75H+ S52- + 4H2O ) 4.00HS- + 1.00SO42- + 4.00H+ S62- + 5H2O ) 4.75HS- + 1.25SO42- + 5.25H+ S72- + 6H2O ) 5.50HS- + 1.50SO42- + 6.50H+ S82- + 7H2O ) 6.25HS- + 1.75SO42- + 7.75H+

77.41 71.63 67.41 66.05 67.15 70.45 73.58

-5.13154 60.8769 125.3254 186.9138 246.0423 302.9708 360.0692

0.8993 -10.6689 -21.9638 -32.7574 -43.1199 -53.0969 -63.1036

HSn-

reaction

∆G°f(HSn-)

∆G°r(HSn-)

log K(HSn-)

2 3 4 5 6 7 8

HS2 + H2O ) 1.75HS + 0.25SO4 + 1.25H HS3- + 2H2O ) 2.50HS- + 0.50SO42- + 2.50H+ HS4- + 3H2O ) 3.25HS- + 0.75SO42- + 3.75H+ HS5- + 4H2O ) 4.00HS- + 1.00SO42- + 5.00H+ HS6- + 5H2O ) 4.75HS- + 1.25SO42- + 6.25H+ HS7- + 6H2O ) 5.50HS- +1.50SO42- + 7.50H+ HS8- + 7H2O ) 6.25HS- + 1.75SO42- + 8.75H+

22.07 28.84 31.47 33.53 37.31 42.44 46.94

50.20846 103.6669 161.2654 219.4338 275.8823 330.9808 386.7092

-8.7992 -18.1681 -28.2624 -38.4567 -48.3495 -58.0057 -67.7724

H2Sn

reaction

∆G°f(H2Sn)

∆G°r(H2Sn)

log K(H2Sn)

2 3 4 5 6 7 8

H2S2 + H2O ) 1.75HS- + 0.25SO42- + 2.25H+ H2S3 + 2H2O ) 2.50HS- + 0.50SO42- + 3.50H+ H2S4 + 3H2O ) 3.25HS- + 0.75SO42- + 4.75H+ H2S5 + 4H2O ) 4.00HS- + 1.00SO42- + 6.00H+ H2S6 + 5H2O ) 4.75HS- + 1.25SO42- + 7.25H+ H2S7 + 6H2O ) 5.50HS- +1.50SO42- + 8.50H+ H2S8 + 7H2O ) 6.25HS- + 1.75SO42- + 9.75H+

-6.46 4.88 9.79 13.56 18.14 23.9 28.91

78.73846 127.6269 182.9454 239.4038 295.0523 349.5208 404.7392

-13.7992 -22.3671 -32.0619 -41.9565 -51.7091 -61.2550 -70.9322

2-

-

-

4

2-

2-

+

+

a ∆G°f(HS-) ) 12.05 kJ mol-1,81 ∆G°f(H2O) ) -237.14 kJ mol-1,84 ∆G°f(SO42-) ) -744.0 kJ mol-1,84 and ∆G°f(Sn2-), ∆G°f(HSn-), and ∆G°f(H2Sn) were taken from Kamyshny et al.78

Figure 11. Figure 7 with stable phases removed, showing polysulfide distribution in terms of pH and Eh. Figure 10. Polysulfide distribution in terms of pH in the presence of excess S(0) calculated according to the Kamyshny et al.78 data set. Modified from ref 1, Copyright 2005, with permission from Elsevier.

most important species in this pH range in this system are S42-, S52-, and S62-. Removing stable sulfide species and rhombic sulfur from the pH-Eh diagram in Figure 7 reveals the underlying metastable polysulfide distribution (Figure 11). The data show the surprising result that disulfide species, S2(-II), dominate much of the pH-Eh space, which would otherwise be occupied by stable S(-II) species. Higher order Sn(-II) species shadow but extend the stability space of rhombic sulfur. In a key area for natural environmental systems, around the SO42-/S(-II) redox boundary at pH 5-9, the dominant polysulfide species are S82-, S62- and HS2-. Rickard and Morse1 commented that one of the features of the Kamyshny et al.78 data set is the remarkable relative stability of the hydrodisulfide ion, HS2-, over the environmentally significant pH range of 6-8. It is the dominant

polysulfide at pH < 7 and contributes to ca. 1% of the total dissolved sulfide in much of the system. The Kamyshny data set can be used to determine the equilibrium distribution of polysulfides in the absence of S(0), using reactions like

S22- + H2O ) 1.75HS- + 0.25SO42- + 0.25H+ (7) This approach is useful for both determining the distribution of polysulfides in much of the natural environment where sulfur is not present or measured and using commercial equilibrium speciation programmes, such as Geochemist’s Workbench (GWB), where reactions are written in terms of the basis species SO42-. Equilibrium constants for polysulfides in terms of reactions like eq 7 are listed in Table 3. The data are calculated via ∆G°f values since the uncertainties in the transformations of measured equilibrium constants to ∆G°f is of the same magnitude as the uncertainty in summing the logarithm of the equilibrium constants for the series



Figure 12. Polysulfide distribution for various pH values at ∑{S} ) 10-3.

of reactions involving SO42-, S0, and HS-. The ∆G°f values for polysulfides are taken from Kamyshny et al.;78 ∆G°f(SO42-) ) -744 kJ mol-1; ∆G°f(HS-) ) 12.05kJ mol-1. The distribution of polysulfides in terms of Eh for pH values between 4 and 10 and 10-3 total sulfur activity is shown in Figure 12. These diagrams are calculated using the REACT algorithms of the GWB suite. Sulfate is reduced by titrating electrons at fixed pH values. As the electron activity increases, the partial pressure of H2 in equilibrium with H2O increases until it reaches 1 bar pressure. Although disequilibrium in water reduction is common, with the formation of considerable overpotentials, this is conventionally regarded as the lowermost potential at which water can remain stable in Earth surface environments for any meaningful period.85 Obviously as these high H2 pressures are approached, the system becomes progressively more difficult to balance and no solution to the equilibrium matrix is possible. Figure 12 essentially shows sections through Figure 11 at constant pH. The diagrams show that the total activity of polysulfide species approaches 10-12 at pH < 7 and constitutes about 10 parts per million of the total sulfide activity. The polysulfide activity reaches a maximum close to where SO42-/S(-II) approaches 1 which, in the environmental literature, appears to be often assumed to approximate the anoxic-oxic (or, more vaguely, oxidized/reduced) boundary zone in low-temperature natural systems. At pH > 7, the polysulfide activity reaches around 10-15 and the polysulfides make up about 0.1 parts per billion of the total sulfide. As shown in Figure 12, the dominant polysulfides are the disulfides, H2S2, HS2-, and S22-. A significant aspect of these computations is that these disulfides display significant equilibrium activities even in the absence of elemental or rhombic sulfur. In the presence of sulfur, the higher polysulfides become the dominant species near to the SO42-/ S(-II) boundary, as was shown in Figure 11.

The activities of the polysulfide species are directly proportional to the activities of the sulfide species. For example, we can write

S0 + HS- ) HS2-

(8)

for which the equilibrium constant, log K ) -11.46, is a direct measurement of {HS2-}/{HS-}. In terms of concentrations in solutions with ionic strengths up to around 0.7, this would still be a reasonable guide. The activity coefficient, computed through Debye-Hu¨ckel-based algorithms, of -1 species is around 0.7 at this ionic strength and about twice that of -2 species. The conversion of activities to concentrations would mean that [HS2-]/ [HS-] ≈ 0.5{HS2-}/{HS-} at I ≈ 0.7. Solutions of polysulfides are mixtures of Sn(-II), S(-II), and S(0) species and their protonated forms. Kamyshny et al.86 showed that polysulfides exchange sulfur isotopes with other sulfur species in the system in characteristic times of SO42-, and are discussed below. The difference between the Fe(III) solubility in inorganic media such as NaCl or NaClO4 at seawater ionic strength (I ) 0.7) and seawater itself is shown by inspection of the measured stability constants for the proposed complexes in Table 5. The values of log β1 and log β2 are similar to those in inorganic media, but the value of log β3 is higher. The result is due to the formation of Fe(III) organic compounds. The result is significant because the experimentally measured solubility of Fe(III) in inorganic media (Figure 13) reaches a minimum around seawater pH, where Fe(OH)30 may be a dominant Fe(III) hydroxy species. The formation of organic Fe(III) complexes therefore becomes particularly important in ocean water Fe chemistry. As shown in Figure 13, the measured concentration of dissolved Fe(III), or at least that fraction that is 106, depending on the configuration of H2O around the FeS nanoparticle. This density discontinuity should provide a means of discriminating between dissolved species and nanoparticles, but this has not been achieved yet in the FeS system. Thus, aqueous FeS clusters are defined operationally in terms of their voltammetric characteristics.166 Rickard178 measured the intrinsic solubility of mackinawite in aqueous systems, and this led to the determination of the stability constant of the monomer, FeS0. However, the maximum concentration of FeS(aq) in equilibrium with mackinawite is 10-5.88 M, in terms of the monomer, and this is at or below the detection limit of most physical-chemical probes available at present. Aqueous FeS clusters, defined operationally in terms of their voltammetric characteristics, may make up a substantial fraction of the sulfide budget of many natural aqueous and sedimentary environments, including lakes,177,180,186,195 river waters,192 wetlands,196 estuarine sediments,2,183,197 marine sediments,196,198,199 and deep ocean hydrothermal vents.200 The distribution of iron sulfide complexes and clusters in terms of the most probable species, FeSH+, FeS0, Fe2+, and the iron hydroxy species described above and using the listed stability constants, is shown in Figure 21. This MINEQL+ computation shows that, with these figures and in the presence of excess S(-II), FeS0 becomes the dominant species above pH ) 7.5 and FeHS+ in more acid solutions. The hexaqua Fe2+ has a minor importance in sulfidic systems and becomes more significant at very acid pH values. The significance of FeS0 in these pH regions is consistent with the electrochemical observation of the widespread distribution of FeS(aq) in marine systems noted above. Even with these limited data for iron sulfide complexes, we would predict that Fe-rich, polynuclear clusters should occur in systems where iron is enriched relative to sulfide, such as some fluvial or lacustrine systems, which again is consistent with observed natural data. The distribution of iron sulfide complexes in the seawaterlike matrix defined above is shown in Figure 22. In this system, we have used high S(-II) activities of 10-3 and high Fe(II) of 10-6, approaching millimolar sulfide and micromolar total iron. The computation is made with the GWB REACT algorithm with all minerals suppressed and with the HS-/SO42- redox couple disabled. Figure 22a shows that

Figure 22. The distribution of iron sulfide complexes in the seawater-like matrix. In this system with high ∑{S(-II)} ) 10-3 and high ∑{Fe(II)} ) 10-6, FeS(aq) dominates Fe speciation and constitutes ca. 1% of the total S activity in the environmentally important pH range around neutral.

FeS(aq), calculated as the monomer FeS0, is the dominant Fe species in the important marine pH region of 6.5-8.5. At higher pH, iron(II) hydroxyl species become important. At lower pH, the FeHCO3+ species is more significant than FeSH+ or Fe2+. This is interesting since sulfate-reducing prokaryotes, which provide the sulfide in these systems, produce 2 mol of carbonate for each mole of sulfide. So a significant HCO3- concentration with consequent Fe complexation is not surprising. The close relationship of microbially produced carbonates and sulfides in these systems has been widely described.201 Fe2+(aq) becomes the dominant Fe species at pH < 5.5 and constitutes 6. The results explain the observation by Rickard and Morse1 that FeSm has not often been reported from direct observations of normal marine sediments, although pyrite is widely distributed. Most observations of FeSm in sediments are from marginal environments where the total concentration of Fe(II) is high. In contrast, as noted above, the FeSaq cluster has been widely observed, which is in accord with these solubility observations. The idea that FeSm is relatively insoluble in sulfidic sedimentary environments is an oversimplification. Compared with Fe solubility in oxic ocean water, the concentration of dissolved Fe(II) in sulfidic systems in equilibrium with FeSm is substantial. The data suggest that Fe(II) is transportable in solution at quite significant concentrations in sulfidic sediments in the presence of FeSm, mainly in the form of an aqueous FeS species. This means that in the global Fe cycle, the transfer of Fe from a normal oxic oceanic environment to an anoxic sulfidic system with FeSm may result in a sharp increase in the dissolved Fe concentration and a consequent increase in the transport of Fe within the system. The result contributes to the understanding of the formation of pyrite concretions in sediments227 and also suggests a flux of Fe from sulfidic sediments with FeSm to more oxic overlying systems. However, the availability of the hexaqua Fe(II) ion itself, which may be significant biologically, may be correspondingly reduced in high sulfide systems.200

5.6. Kinetics and Mechanism of Mackinawite Formation in Aqueous Solutions The kinetics and mechanism of mackinawite formation in aqueous solutions at low temperatures has been studied by Rickard.220 The mechanism involves two competing reactions involving aqueous H2S and HS-. The rate laws for both reactions are consistent with Eigen-Wilkins mechanisms228 where the rate is determined by the exchange between water molecules in hexaqua iron(II) sulfide outer sphere complexes, Fe(H2O)62+‚H2S and Fe(H2O)62+‚HS-, and the inner sphere complexes, FeH2S‚(H2O)52+ and Fe(SH)‚(H2O)5+. The subsequent nucleation of FeS is fast. There is no observable lag phase and, as discussed above, it is probable that aqueous FeS clusters, with the same structures as the fundamental structural elements in mackinawite, are involved. The rate of formation of FeSm is therefore described by two competing reactions, one with H2S and one with HS-. For the H2S reaction, the overall reaction can be written

Fe2+ + H2S f FeSm + 2H+

(20)

The rate law is

∂cFeSm/∂t ) k1{Fe2+}{H2S}

(21)

where {H2S} and {Fe2+} are the formally dimensionless H2S and Fe2+ activities, which are represented on a moles per liter scale for practical convenience, and k1 is the rate constant where log k1 ) 7 ( 1 L mol -1 s -1. Rickard220 showed that the reaction involving HS- resulted in the formation of an intermediate complex, [Fe(SH)2], which then condenses to FeSm. The lack of thermodynamic stability of [Fe(SH)2] is consistent with the data on iron sulfide complexes discussed above.


The two competing mechanisms involved in FeS precipitation mean that the rate is pH dependent. The rate of the H2S pathway becomes equal to and greater than that of the HS- pathway as [S(-II)]T reaches 10-5 M or less under nearneutral conditions. In sulfide-rich environments (i.e., with micromolar or greater S(-II) concentrations), the rate of sulfide removal is 2 orders of magnitude greater in neutral to alkaline solutions than in acid environments. By contrast, in sulfide-poor systems, the rate is greater in neutral to acid conditions. As discussed above, it is possible that the major reactive iron phases involved in FeSm formation in marine environments are iron hydroxides. As we show, it is possible that the reactive phases include mixed FeIIFeIII hydroxides related to the green rust group. However, at present it is still generally assumed that iron(III) oxyhydroxides, probably in the form of nanoparticulate goethite, are the main phases. Little work has been reported on the sulfidation of the green rusts, although this reaction was noted in 1969.209 In contrast, the kinetics and mechanism of the reaction between goethite and sulfide have been reported.229-231 The product of this reaction is FeSm and colloidal sulfur. The rate is dependent on pH, [S(-II)]T, and the surface area of the goethite, and the kinetics are consistent with the formation of a sulfide complex at the oxyhydroxide surface followed by electron transfer between the sulfide and Fe(III). Sulfide free radicals are proposed to be formed, which would react with Fe(III) to form elemental sulfur, the Fe(II) then reacting with the S(-II) to form FeSm, as in the Rickard kinetics described above. Biber et al.232 showed that the reaction kinetics are affected by organic and inorganic species, with phosphate being particularly important. The key reaction in the formation of FeSm from aqueous solutions is the reaction between Fe(II) and S(-II), independently of whether the reactive iron phase is hexaqua Fe(II) directly or iron(III) (oxy)hydroxide. As discussed with respect to the reduction of sulfate, the form of the Fe involved in the reaction with S(-II) to form FeSm is influenced by the prevalence of anaerobic dissimilatory Fe(III) ironreducing prokaryotes (FeRP) in the sediment column.233-240 Many of the Fe(III)-reducing organisms are closely phylogenetically related to sulfate-reducing bacteria and some species of the Geobacteraceae reduce S(0). The iron-reducers and sulfate-reducers may be part of a tight ecology, since the acetate used by the FeRP is a common product of some strains of SRP. Kostka and his associates showed that the Fe(III) reducers not only reduced iron(III) oxyhydroxides but also were able to reduce iron(III) sheet silicates.241-244 Kostka and Nealson245 showed that the Fe(III) reducers could also reduce the Fe(III) in magnetite. The significance of these observations is that magnetite and the Fe sheet silicates are usually classified as poorly reactive or unreactive iron phases.42-44 As with the SRP, these organisms bring a sophisticated armory of enzymatic catalysts to bear on marine Fe chemistry such that, counterintuitively, Fe equilibrium in the presence of FeRP may be more nearly attained. The net results of the ubiquity of FeRP and their close association with SRP in marine sediments is that the process of formation of iron sulfides like FeSm in marine sediments may not involve a reaction with solid Fe minerals like goethite but may include direct reactions between hexaqua Fe(II) formed by FeRP and sulfide produced by SRP.

Rickard and Luther

5.7. Kinetics and Mechanism of Mackinawite Dissolution in Aqueous Solutions The rate of FeSm dissolution has been investigated by Pankow and Morgan246 who showed a first-order dependence on H+ concentration in acid solutions and a constant H+ concentration independent rate in neutral to alkaline solutions. These kinetic data are consistent with the solubility data of Rickard178 discussed above, which demonstrated that in neutral to alkaline solutions, FeSm solubility was dominated by FeS0 in a pH-independent reaction whereas, in more acid solutions, the solubility was determined by the activities of Fe2+ and H2S. The rate equation in systems with no added Fe or S can be expressed as

-∂FeSm/∂t ) k1[H+] + k2

(22)

where k1 ) 0.18 ( 0.06 cm min-1 and k2 ) (1.9 ( 0.9) × 10-9 mol cm-2 min-1 and the rate is expressed in units of mol cm-2 min-1. Simple inspection of eq 22 shows that the H+ concentration independent rate dominates the dissolution at pH > 5.3, which is consistent with the solubility data of Rickard.178 The rate in most low-temperature, natural aqueous solutions such as seawater is therefore given by

-∂FeSm/∂t ) k2 Pankow and Morgan246 expressed the rate of dissolution in solutions containing Fe and S in terms of the function (1 - c/cs) where c is the concentration of Fe(II) and S(-II) in solution and cs is the concentration at equilibrium with FeSm. This is a version of the classical (1 - Ω)n function for dissolution reactions247 where Ω is the degree of supersaturation. Pankow and Morgan246 assumed that n ) 1. Then eq 22 becomes

-∂FeSm/∂t ) k2(1 - Ω)

(23)

The maximum rate of FeSm dissolution occurs far from equilibrium where Ω is small. Pankow and Morgan’s246 data suggest that this rate approaches 3 × 10-7 mol m-2 s-1. The specific surface area for FeSm has been established by Ohfuji and Rickard194 to be 380 m2 g-1. This suggests that FeSm dissolves at a rate of up to 10-4 mol s-1. The data show that FeSm reaches equilibrium rapidly with dissolved Fe and S. In order for FeSm to be preserved for any length of time, Ω f 1. Pankow and Morgan246 found that the Arrhenius energy for k1 was 28 kJ mol-1 and that for k2 30 kJ mol-1. Within the uncertainties of the measurements, these are very similar and within the range expected from transport-controlled reactions. The observation implies that the rates of the surface chemical reactions involved in FeSm dissolution are so fast that the rate-limiting factor is diffusion of the components away from the surface. The rate law (eq 23) suggests that the rate close to equilibrium will become very slow and the rate-controlling reaction may change. The rate of FeSm dissolution under these conditions has not been investigated, however, and the current data refer to dissolution far from equilibrium.

6. Iron Thiospinel, Greigite Greigite is the thiospinel of iron, Fe3S4g. Skinner et al.248 originally defined greigite from a tertiary lacustrian sequence



Figure 30. Greigite structure. Greigite is an inverse spinel, A(AB)2S4 where FeII atoms occur in tetrahedral A-sites and mixed FeIII and FeII occur in the octahedral B-sites. Modified from ref 1, Copyright 2005, with permission from Elsevier.

in California. Earlier Polushkina and Sidorenko224 described an iron thiospinel and named it melnikoVite after a ferromagnetic iron sulfide reported in 1912 by Doss249 from the estates of Count Melnikoff. Unfortunately, Doss described melnikoVite as a magnetic variety of FeS2. Ramdohr250 used the term melnikoVitic pyrite for a black iron sulfide mixture found in some hydrothermal ore deposits. Rickard209 showed that melnikoVitic pyrite was a mixture of pyrite, mackinawite, greigite, and some iron oxyhydroxides. Because of the confusion surrounding the term melnikoVite, the International Mineralogical Association approved greigite as the official name for the thiospinel of iron.

6.1. Greigite Structure Greigite is the sulfur analogue of magnetite and has a similar inverse spinel structure (Figure 30). The unit cell of greigite has a ) 9.876 Å and consists of eight Fe3S4 moieties. Eight Fe atoms occur in tetrahedral A-sites and 16 in octahedral B-sites. In the spinel structure, this arrangement would reflect the formula AB2S4 where A ) Fe(II) and B ) Fe(III) giving the overall formula for greigite, FeIIFeIII2S4. However, molecular orbital251 calculations suggested that the Fe in the octahedral B-sites is mixed FeIII and FeII. This would tend to an inverse spinel structure, A(AB)S4, and may suggest nonstoichiometry in greigite. The greigite structure can be regarded as a cubic, closepacked array of S atoms linked by smaller Fe atoms. This arrangement shows a remarkable congruity with the cubic, close-packed S array of mackinawite, which was first pointed out by Yamaguchi and Katsurai.252 Projections of both

Figure 31. Homology of the mackinawite and greigite structures (based on Lennie et al.211) Projections of both structures onto (001) emphasize the similarity in the close-packed cubic arrays of S -atoms in both minerals. Modified from ref 1, Copyright 2005, with permission from Elsevier.

structures onto (001) emphasize the similarity in structural arrangements (Figure 31). The implication is that the transformation of mackinawite to greigite is essentially a


Figure 32. High-resolution transmission electron micrograph of a greigite grain showing relict mackinawite interlayers made by Posfai et al.253 The grain is from a bacterial magnetosome. The arrows delimit bands several atomic layers in thickness parallel to (222), which retain relict mackinawite structural elements. The cubicclosed packed S array is continuous across the interface between the two materials. Reprinted with permission from Science (http:// www.aaas.org), ref 253. Copyright 1998 American Association for the Advancement of Science.

rearrangement of Fe atoms in a close-packed, cubic array of S atoms. The rearrangement is caused by the oxidation of two-thirds of the mackinawite Fe(II) to greigite Fe(III), where the Fe(III) acquires an octahedral coordination. The transformation involves a reduction in the volume of the cubic, closed-packed S array of 12% in the transformation.211 This is consistent with greigite being more stable than mackinawite. The homology of the mackinawite and greigite structures was originally suggested by Yamaguchi and Katsurai204 and Lennie et al.211 High-resolution electron microscopy of the transformation (Figure 32) has neatly confirmed the crystallographic considerations regarding the structural congruity of mackinawite and greigite originally suggested by Yamaguchi and Katsurai252 and Lennie et al.211 The HR-TEM study does bring up one worrying aspect about the dependence on XRD analyses to determine mackinawite and greigite. Figure 32 shows mackinawite interlayers within a greigite crystal. At these dimensions, it is doubtful whether the mackinawite interlayers would be detected by conventional XRPD, although Yamaguchi and Katsurai252 assigned the asymmetry of the {200} greigite XRPD peak to FeSm contamination. The problem is not simply a matter of concentration but also of the size and lack of continuity of the mackinawite layers. It is presently not easy to detect relict mackinawite within greigite. The result is that greigite analyses are prone to uncertainties due to the presence of FeSm in the structure within concomitant effects on both compositional and solubility analyses.

6.2. Greigite Composition The composition of greigite is not well-constrained. There have been few recent reports of total analyses, mainly because of the difficulty of separating greigite from related

Rickard and Luther

iron sulfides and (oxyhydr)oxides and the difficulties of sufficiently accurate S analyses on small amounts of material. Skinner et al.248 originally reported Fe3.00S4.00. Polushkina and Sidorenko,224 in contrast, gave the composition as ranging from Fe2.83S4 to Fe3.11S4. Some reports have been presented using electron beam methods to analyze greigite or greigite-containing particles. The problem here is that the analyses at best provide Fe/S ratios with an uncertainty that would not preclude nonstoichiometry and totals are difficult to obtain. Spender et al.254 suggested that greigite was nonstoichiometric because Mo¨ssbauer spectra did not yield the expected intensity ratio of 1:2 of the spinel A and B sites. B-site vacancies would lead to nonstoichiometry. Dekkers et al.255 and Posfai et al.256 noted that this effect could be enhanced by storage of greigite samples but note that nonstoichiometry could explain the lack of any lowtemperature transition in greigite analogous to the Verwey transition in magnetite. The suggestion that the Fe in the octahedral sites contain mixed Fe(III) and Fe(II)251 again might suggest at least a propensity for nonstoichiometry. Rickard and Morse1 concluded that greigite probably does display some degree of nonstoichiometry. Greigite has been reported to contain small amounts of Cu, Co, and Ni. However, Posfai et al.256 reported that mackinawite and mackinawite-greigite mixtures contained significant Cu but an apparently pure greigite crystal contained little Cu. The amounts of Cu are not listed here because the analytical precision was necessarily limited in these analyses. However, as Posfai et al.256 point out, mackinawite from the high-temperature monosulfide solid solution association can incorporate large concentrations of Cu into its structure. The solid-state transformation of mackinawite to greigite would therefore tend to include varying amounts of inherited Cu in transitional stages. However, it is still not clear whether greigite can contain significant (e.g., >1 wt %) Cu in its structure. The valence electrons in greigite are localized. Analogous thiospinels of Co, Ni, and mixed thiospinels of Cu and Co and Fe and Ni have delocalized valence electrons. Vaughan and Craig213 suggested that this is the reason that there is no stable solid solution between greigite and Co, Ni, and Cu thiospinels. This would limit the amount of Cu, Ni, and Co that could be contained in the greigite structure.

6.3. Greigite Solubility The net reaction of greigite with mineral acids can be written

Fe3S4 + 6H+ ) 3Fe2+ + 3H2S + S(0)

(24)

The reaction shows elemental sulfur being produced at a ratio of 1 mol of S(0) for each mole of Fe3S4 digested. Cornwell and Morse222 found that this accorded with what they observed in the laboratory. They found that reaction with cold 1 N HCl, cold 6 N HCl, cold 6 N HCl + SnCl2, or hot 6 N HCl resulted in visually complete digestions of greigite but they only recovered 75% of the S(0). Complete recoveries (i.e., 93-100%) were only possible in the presence of a strong reducing agent such as Sn(II) or Ti(III). Filtration of the digestion and treatment with Cr(II) resulted in recovery of a further 25% S, strongly suggesting that the missing S is in the form of S(0). The problem with the purity of the synthetic greigite also affects the measurement of the solubility of greigite in



Table 10. Recalculation of Berner’s225 Experimental Measurements of the Total Fe2+ Concentration in Equilibrium with Greigite at 25 °C Using Thermodynamic Data Cited in the Texta pH

[Fe2+]

γFe2+

{Fe2+}

log {HS-}

log K

∆G°f(Fe3S4), kJ mol-1

∆G°f(Fe3S4), kcal mol-

Berner, kcal mol-1

2.93 2.92 2.97 2.93 2.99 2.95 2.92 4.01 4.04

1.30 × 10-2 1.10 × 10-2 1.20 × 10-2 1.20 × 10-2 9.50 × 10-3 9.60 × 10-3 1.10 × 10-2 8.30 × 10-5 7.30 × 10-5

0.5082 0.4996 0.5258 0.5138 0.5156 0.5102 0.4996 0.9139 0.9051

0.0066 0.0055 0.0063 0.0062 0.0049 0.0049 0.0055 0.0001 0.0001

-5.0490 -5.0590 -5.0090 -5.0490 -4.9890 -5.0290 -5.0590 -3.9690 -3.9390

-12.90 -13.20 -12.72 -12.99 -12.93 -13.17 -13.20 -12.24 -12.24

-308.62 -310.33 -307.59 -309.13 -308.79 -310.16 -310.33 -304.85 -304.85

-73.76 -74.17 -73.52 -73.88 -73.80 -74.13 -74.17 -72.86 -72.86

-69.60 -70.00 -69.30 -69.70 -69.60 -69.90 -70.00 -68.70 -68.70

-12.84

-308.30

-73.68

-69.50

average a

The Fe2+ activity coefficient, γFe2+, is calculated from Berner’s data. The calculated standard free energy for greigite, ∆G°f(Fe3S4), is also listed in kcal mol-1 and compared with Berner’s original value.

mineral acids. Cornwell and Morse222 used Wada’s257 recipe for preparing greigite by boiling FeSm with 5 mL of polysulfide solution overnight. They noted that aging of greigite in solution for 1 week decreases its dissolution in weak acids. This is in accord with TEM observations, which suggest that mackinawite continues to transform to greigite on storage. Cornwell and Morse222 concluded that it was not possible to separate greigite from pyrite via acid digestions. Allen and Parkes223 used Berner’s225 recipe for synthesizing greigite. They found recoveries of 34% ( 12% in cold 6 N HCl and 88% ( 4% in hot AVS digestions. With CrCl2 added, the resulting recoveries were 91% ( 24% and 78% ( 21%, respectively. The worrying aspect of these results is not only the deviations from 100% recovery but also the magnitude of the standard deviations. These imply a considerable lack of reproducibility or uncertainty in the results. The analysis of greigite in a mixture of iron sulfides remains a primary hindrance to progress on understanding greigite formation. Although a number of analytical protocols have been suggested for mixed iron sulfide analyses, none of these are sufficiently accurate in the presence of greigite.222,258-264 The problem is that the quantification of greigite dissolution is difficult. Ideally, greigite should dissolve in acid to produce S(-II) and S(0). However, the stoichiometry of this reaction is difficult to control, especially in a mixture including pyrite. In experimental systems, analysis of the Fe(III) contents of iron sulfide mixtures may be possible since greigite is the only common iron sulfide containing Fe(III). However, this is precluded in natural systems because of the presence of Fe(III) in a number of associated Fe minerals. The solubility of greigite has only been measured once, by Berner.225 Berner synthesized greigite by bubbling H2S through an aerated FeSO4 solution at 80-90 °C for around 45 min. Berner assumed that the system was in equilibrium with rhombic sulfur, although this was not identified. Using this assumption, the solubility could be calculated from the reaction

Fe3S4 ) 3Fe2+ + 3S2- + S0

(25)

where S0 is rhombic sulfur. Berner took Maronny’s76 value for pK2(H2S) of 13.9 and reported the solubility in terms of ∆G°f using free energy values from Garrels and Christ.85 His original value for ∆G°f(Fe3S4g) was -69.4 ( 0.7 kcal mol-1 (i.e., -290.4 kJ mol-1). We recalculate this value from Berner’s original listing of analyses of total dissolved Fe2+

in equilibrium with greigite (Table 10) and using Berner’s original activity coefficients for Fe2+ calculated from his data. The reaction is rewritten in terms of HS-, in order to avoid the uncertainty in pK2(H2S),

Fe3S4 + 3H+ ) 3Fe2+ + 3HS- + S0

(26)

and the thermodynamic data used are listed in this paper. The result is that the free energy of formation of greigite is -308.30 kJ mol-1 and log K for reaction 26 is -12.84. This result compares with Berner’s original result of -290.4 kJ mol-1, suggesting that greigite is a little more stable than originally supposed. An alternative value recalculated from Berner’s measurement of -273.8 kJ mol-1 has been published,101 but the way that this value was recalculated was not shown and the authors appear to have used the erroneous NBS ∆G°f(Fe2+) value as discussed above. This value would suggest a substantial decrease in greigite stability. The problem is that it is difficult to synthesize pure greigite. Berner used XRPD methods to demonstrate that the product was mainly greigite, but subsequent high-resolution electron microscopic analyses of greigite precipitates have commonly shown that a varying amount of FeSm often occurs in greigite particles. Since mackinawite is more soluble than greigite, the effect would have been to increase the apparent solubility of greigite. The suggested greigite solubility based on Berner’s original measurement is therefore likely to be too high, and the actual solubility of greigite is probably less than these numbers. This means that greigite is more stable in the Fe-S-H2O system than the solubility measurements would imply. It is obvious that it is important for understanding of the sedimentary iron sulfur system that greigite solubility be revisited. Severe technical difficulties must be overcome, however. Pure greigite needs to be synthesized, its composition needs to be established, and a means of accurately determining the activities in solution needs to be found.

6.4. Reactions To Form Greigite Greigite was synthesized by Yamaguchi and Katsurai252 before the material was identified naturally. Since that time a number of recipes have been tried for greigite synthesis (Table 11). The interesting aspect of these syntheses is the varying conditions. Thus elevated temperatures, S(0), and O2 are not necessary for greigite syntheses, although these


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Table 11. Greigite Recipes reactants

a

T (°C)a

FeSm FeSm FeSm + H2S(g) + aldehydic carbonyl Fe(NH4)2(SO4)2 + Na2S + S(0)

25+ 200 40

Fe(NH4)2(SO4)2 + Na2S + sodium polysulfide Fe(NH4)2(SO4)2 + Na2S Fe(NH4)2(SO4)2 + Na2S (pH ) 5) Fe(NH4)2(SO4)2 + Na2S (pH ) 3) FeSO4 +H2S(g) FeSm ( H2S (g) FeSO4 ( Na2S

comments

ref 265 204 273

100

under TEM beam anhydrous aldehydic carbonyl recovered after reaction Fe(NH4)2(SO4)2 injected into Na2S + S(0) mixture at 140 °C pH ) 5.8-6.1

190 150

FeSm precipitated FeSm precipitated

268, 266, 274 275

100

FeSm precipitated; H2SO4 added and product reheated to 80 °C aerated FeSO4 at pH ) 3 FeSm freeze-dried and exposed to air pH > 3.4-6.5

275

140

80 35-160 25

255 257

225 267 209

Unless otherwise stated, the reactants were mixed at room temperature and heated to around the listed temperature for various periods.

may help in the process. Current work in the Rickard laboratory suggests that greigite may form directly through the reaction between mackinawite and anoxic H2O (i.e., in the absence of both aqueous O2 and excess S(-II)). The reaction appears to be quite rapid at temperatures somewhat above 70 °C. This observation is consistent with earlier speculations that greigite may have an increased stability in this temperature regime. However, the reaction does not appear to balance at first sight in the absence of an oxidizing agent unless it is autocatalytic. One factor that seems to be consistent is pH. Thus, Yamaguchi and Wada268 (p 477) wrote, “It is known that a high concentration of hydrogen ion promotes formation of greigite.” The most successful syntheses are carried out in acid pH systems with pH values as low as 3 being used. It does not appear that greigite has ever been reported to form directly from solution. All syntheses of this material involve pre-existing mackinawite, which is metastable with respect to greigite.225 The observations of the apparent requirement for pre-existing FeSm and the acid pH are consistent with observations in natural systems where greigite is commonly observed in freshwater sediments, which generally are more acid than marine systems. The pH dependence might be mechanistic or simply reflect enhanced dissolution of FeSm in more acid environments, giving faster reaction rates and the removal of the FeSm reactant. However, the kinetics and mechanism of the greigite formation reaction have not been studied in aqueous solutions. Until this is done, the present conclusions about the reaction mechanism must remain somewhat empirical. Greigite has also been synthesized microbiologically. Freke and Tate269 reported greigite from SRP enrichment cultures, although Rickard270 found no greigite in Freke and Tate’s samples. They may have oxidized by the time Rickard analyzed them (cf. ref 271), but he found only magnetite and FeSm. Freke and Tate269 thought the sample contained greigite because it was ferromagnetic and contained Fe and S. The use of SRP enrichment cultures to produce greigite has been promoted by Watson and his group as a possibly means of cleaning up nuclear waste.272 Rickard270 synthesized greigite with pure cultures of the SRP DesulfoVibrio desulfuricans. Posfai et al.256 described the iron sulfides formed in magnetotactic bacteria. They found both mackinawite and greigite to be common and showed that the greigite developed from a solid-state transformation of mackinawite. Rickard et al.273 showed that trace amounts of aldehydic

carbonyls enable greigite formation from mackinawite and inhibited the formation of pyrite. This work suggests how the bacteria may produce greigite (rather than pyrite) from mackinawite, since low concentrations of aldehydic carbonyls (and by inference, other as yet unknown organic moieties) are involved in key biochemical processes in the form of glyceraldehydes, for example. Detailed electron diffraction studies211,256,257,274,275 and highresolution synchrotron XRD211 of the mackinawite to greigite transition in anhydrous environments demonstrate that this is a solid-state transformation with a structural congruency between the two phases. Greigite formation requires the oxidation of two-thirds of the Fe(II) of mackinawite to Fe(III), whereas the S(-II) is not oxidized. In contrast, pyrite formation from iron(II) monosulfide requires that the FeS Fe(II) remains unoxidized whereas the S(-II) is oxidized (see below). The pyrite reaction involves a dissolved stage, whereas the greigite reaction is a solid state. The results suggest that the oxidation of FeSm S(-II) occurs in solution whereas the oxidation of FeSm Fe(II) occurs in the solid state. Lennie et al.211 showed that the greigite XRD reflections were first detected at 100 °C after stepwise heating of mackinawite from room temperature. Transformation was complete after 5 h at 200 °C, and greigite began to decompose at temperature beyond 260 °C. A similar result was found using transmission electron microscopy. In the XRD and electron microscope experiments, no water was present and the atmosphere was the machine vacuum. No other components were present, and no other products were detected. So what happened to the electrons? The reaction stoichiometry, in the absence of any other reactant, would appear to be

4FeSm ) Fe3S4g + Fe(0)

(27)

However, ∆G°r for this reaction is +84.5 kJ mol-1 at 25 °C (see below), and the reaction seems to be thermodynamically improbable. Rickard and Morse1 discussed the electronic balance of the reaction quantitatively. It appears likely that O2 was involved in the reaction and that it was introduced when the FeSm was briefly exposed to air. Posfai et al.256 concluded that the oxidation occurred in the 10 days storage preceding the HR-TEM work. Mullet et al.,276 for example, found that the surface of mackinawite contained 19 at. % O by XPS analyses and ascribed this to oxidation during sample



handling. Mullet et al.’s mackinawite was prepared in the same manner as that of Lennie et al.,211 and they described their reactant as 1-2 µm tabular FeS crystals. Mullet et al. did not report greigite formation, and the 19 at. % surface oxygen that they analyzed would provide insufficient O2 to produce measurable amounts of greigite if modeled as a monolayer on the mackinawite surface. To produce measurable (g1 wt %) quantities of greigite, some 0.3 wt % of the reactant mackinawite needs to be oxidized in situ. The reaction of dry mackinawite with air has been followed by Boursiquot et al.271 They showed that greigite, sulfur, and iron (oxyhydr)oxides were formed. After 6 months, greigite itself was entirely converted into sulfur and iron (oxyhydr)oxides. It appears therefore that sample handling in air during the experimentation and analysis is sufficient to provide enough oxidation to account for the formation of greigite reported in several accounts of anhydrous FeSm reaction chemistry.101,211 The reaction is accelerated under electron and X-ray beams probably due to damage to the FeSm lattice under the influence of the beam and then a structural rearrangement to the lower energy state of greigite. The net stoichiometry of the reaction involving O2 may be written

4FeS + 0.5O2(g) ) Fe3S4(greigite) + FeO*(s)

(28)

where FeO*(s) represents an unspecified (oxyhydr)oxide of iron. The reaction is thermodynamically favored if ∆G G°f(FeO*) < -84.5 kJ mol-1. Since the old NBS ∆ G°f(wu¨stite) value was around -244 kJ mol-1, this seems thermodynamically possible. The mass balance would suggest that the product should be heavier than the initial FeSm reactant by 0.5 mol of O2 for each mole of Fe3S4 produced. In mass terms, this is an uncertainty of less than 3 wt % in the analysis, well within the analytical error. In other words, the iron (oxyhydr)oxide produced during the reaction would not necessarily be seen. Lennie et al.211 checked the Fe/S ratio of the greigite-mackinawite mixture produced by heating the mackinawite at 100 °C. The average Fe/S ratio of seven such mixed grains was 1.00 ( 0.07, which is close to the original composition of the FeSm reactant. The stoichiometry converts to an analytical uncertainty of around (2 wt %, or adequate to include sufficient (oxyhydr)oxide to account for the electron balance. The reaction of anhydrous FeSm with O2 appears to involve the direct reaction with FeS-FeII rather than the reaction with dissolved S(-II). The development of significant Fe-Fe bonding in mackinawite compared with the aqueous FeS molecular clusters leads to an apparent oxidation number of 0 for the FeSm-Fe. This suggests the strong possibility of rapid oxidation of the Fe by O2 in the condensed phase. An initial reaction, such as that described in reaction 28, with O2 immediately grabbing electrons from Fe(0) in the mackinawite, as suggested by Lennie et al.,211 is another way of presenting this reaction. The concern is, of course, that greigite will form artefactually from mackinawite during XRD and TEM analyses. In contrast, the autoxidation of FeSm in water to form Fe3S4 is not thermodynamically favored at 25 °C. All possible iron oxide, hydroxide, and oxyhydroxide products, as listed in Table 6 give rise to a positive ∆G°r value for reactions like

4FeSm + 2H2O ) Fe3S4 + Fe(OH)2 + H2

(29)

This means that the PH2 in equilibrium with such reactions

Figure 33. pH-Eh diagram showing the mackinawite-greigite boundary at 25 °C, 1 bar total pressure, ∑{Fe}T ) 10-3, and ∑{S(-II)} ) 10-3 and 10-6.

is inhibitingly high. For example, in reaction 29, it would be around 10 atm at 25 °C. This is in accord with observations in the Rickard laboratory that H2 is not detected as a product in the apparent autoxidation of FeSm to form Fe3S4. Of course, the autoxidation reaction is not precluded at higher temperatures. FeSm appears to become more rapidly unstable with respect to greigite above ca. 70 °C. This suggests that reactions like reaction 29 may not display linear temperature dependence. The stability relationships between greigite and mackinawite in “seawater” conditions are shown in the form of a pH-Eh diagram in Figure 33. {Fe}T is set at 10-6, approximating to micromolar Fe concentrations, which are higher than those found in normal seawater. However, at lower {Fe}T, greigite stability becomes insignificant, which is consistent with the scarcity of observations of greigite in normal marine sediments. Note also that these data are computed for the recalculated greigite stability data and the remeasured mackinawite stability, which makes greigite more stable than was previously supposed. For example, for the reaction

3FeSm + S0 ) Fe3S4g

(30)

∆G°r ) -13.7 kJ mol-1. Thus greigite is stable relative to mackinawite plus sulfur, and in acid conditions at high {S}T where sulfur is stable, greigite will form readily from mackinawite, as is observed. Note that the conversion involves the formation of 1 mol of Fe3S4 for 3 mol of FeSm. The pH-Eh diagram (Figure 33) is drawn for {S}T ) 10-3 and 10-6, equivalent to approximately millimolar and micromolar sulfide concentrations. Greigite replaces mackinawite in more acid systems with higher Eh values. This result is consistent with observations that greigite is most abundant in freshwater systems where such conditions may prevail.

7. Iron Disulfide, Pyrite Pyrite, cubic FeS2p, is the most common sulfide mineral in Earth surface environments. It is sometimes said to be


Figure 34. Structural elements of pyrite.

the most common sulfide mineral on Earth, but this is doubtful. Pyrrhotite group minerals are common in mantle rocks and meteorites, and it is probable that pyrrhotite group minerals are the most common sulfides in the bulk Earth. However, the abundance of pyrite on the Earth surface has led to it being the target of many pioneering investigations. Thus, pyrite was the first mineral structure determined in 1914 by Bragg277 with his new X-ray diffraction system. In 1804, Hatchett278 found the composition of pyrite to be FeS2. In 1815, Bakewell279 inadvertently synthesized pyrite serendipitously when a mouse got into a jar of ferrous sulfate in his laboratory. Bakewell examined the mouse droppings and discovered that they were covered in pyrite crystals. Allen et al.280 synthesized pyrite hydrothermally in 1912. Sugawara281,282 first reported the importance of pyrite in marine sediments, and Berner’s206,217,225,283-285 and Rickard’s209,270 work in the 1960s led to the key role of pyrite in the Earth surface system being fully appreciated. In the geochemical literature, there is much made of a “controversy” about pyrite formation, especially with respect to its formation in low-temperature environments such as marine sediments. In fact, critical examination of the literature reveals that no such controversy actually exists. The concept is fueled by (1) a misunderstanding of the various concepts of mechanism and process in some of the geochemical literature; (2) experimental problems with pyrite syntheses, which often require large amounts of components (relative to pyrite solubility) in order to obtain sufficient amounts of product for study; and, of course, (3) the promotion of reports of investigations and grant proposals on the back of the idea that there is a controversy and it is about to be resolved.

7.1. Pyrite Structure The structure of pyrite (Figure 34) is well-known. Pyrite is an iron(II) disulfide with a NaCl-type structure. The S22groups are situated at the cube center and the midpoints of cube edges, and the low-spin FeII atoms (d6, t2g6) are located at the corners and face centers. The arrangement of the disulfide dumbbells is such that the structure, although cubic,

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has a relatively low symmetry, space group Pa3. The structure has 3-fold axes along the 〈111〉 directions and 2-fold axes along the 〈100〉 directions. The 2-fold symmetry means that the [100], [010], and [001] zone axes (equivalent to the a, b, and c crystallographic axes) are crystallographically not interchangeable with each other by a simple 90° rotation as in simple cubes. One result of this structure is that pyrite, along with several other minerals, exhibits chirality. Thus Guevrement et al.286 demonstrated that there are significant differences in the sensitivity of pyrite to oxidation of the (100) and (111) planes. This chirality of pyrite has been theoretically exploited in the involvement of pyrite in the adsorption of organic molecules and, consequently, in prebiotic syntheses implicated in the origins of life.287 However, this idea was challenged by Pontes-Buarque et al.288 who argued that Stern-layer modulation of surface charge, acetate adsorptive behavior and the requirement for divalent cations in the attachment of organic key molecules, make such chiral-discriminator character of pyrite unlikely in this context. Bither et al.289 first presented a molecular orbital (MO) interpretation of the pyrite structure, which has been further described by Tossell et al.,290 Luther,291 and Rickard et al.170 The MO and frontier molecular orbital (FMO) calculations have been shown to have significant implications for a fundamental explanation of pyrite properties and in predicting both bulk and surface reactions.52,292-294

7.2. Pyrite Composition Kullerud and Yoder295 originally suggested that the composition of pure pyrite is stoichiometric FeS2. They concluded that deviations from stoichiometry were caused by analytical uncertainties or the presence of traces of other elements in the material. Ellmer and Hopfner296 used theoretical arguments and a critical review of stoichiometry measurements to show that pyrite has a very narrow homogeneity range ( S42- > HS> HS2- > S32- > H2S. Note that longer chain Sn2- species are actually more nucleophilic than HS-, and HS2- is less nucleophilic than HS-. This means that, although HS2- may be the most abundant polysulfide in many environmentally significant pH regimes, it is less nucleophilic than HS- and likely to react more slowly. These FMO considerations are interesting because they help explain why the addition of excess rhombic sulfur in a sulfide solution is a preferred route for the rapid syntheses of pyrite at low temperatures. As shown in Figure 11, the dominant Sn(-II) species in the S8 stability field are species with n g 5. These longer chain polysulfides are the most nucleophilic species, and thus their relative abundance is expected to enhance the rate of the rate-controlling reaction in pyrite formation, the nucleophilic attack on the [FeS]


Rickard and Luther

nucleophilic attack but is involved in a redox reaction resulting in the oxidation of [FeS]-S2- to S22-. In both reactions, FeS is a nucleophile, but in the polysulfide pathway, it is also an electrophile because higher order and more nucleophilic polysulfides can bind to the Fe(II) in FeS. Rickard54 and Rickard and Luther51 demonstrated that the kinetics of this reaction involved a rate-controlling step involving H2S. They showed that the mechanism involved the formation of an inner sphere complex (Figure 37b) between [FeS] and H2S followed by electron transfer between S(-II) and H(I) to produce S2(-II):

[FeS] + H2S f [Fe-SfSH2] f FeS2 + H2 (52) Figure 37. Proposed pyrite formation mechanisms from soluble FeS and (a) polysulfides and (b) hydrogen sulfide. Sulfur isotope data support these mechanisms.

reaction intermediate. Luther311 reacted pure Sn2- solutions with Fe(II) and independently reproduced the kinetics found by Rickard.309 Figure 37a shows the proposed mechanism, which indicates that the FeS2p formed should have all S atoms from Sn2- species. Butler et al.334 confirmed this isotopically.

where [Fe-SfSH2] is a reaction intermediate. In this reaction, H2S acts as an oxidizing agent with respect to [FeS]. Butler et al.334 confirmed this mechanism isotopically. They demonstrated that the δ34S of the product FeS2p from the reaction equaled a 1:1 mixture of the δ34S of the reactant FeSm and H2S. The rate equation for the H2S mechanism is

∂[FeS2p]/∂t ) k[FeSm][H2S]

(53)

The sulfide-controlled reaction to form pyrite stems from an original observation by Berzelius,335 which was largely forgotten in the 20th century. It was revisited in reports by Wikjord et al.336 and Huber and Wa¨chtershauser337 that H2S reacted with iron(II) monosulfide to form pyrite. The Berzelius reaction is

where [FeSm] is the molar concentration of FeSm (applicable because the reactant FeSm had a comparable initial surface area in all experiments), [H2S] is the molar concentration of H2S, and k is the rate constant.54 Butler et al.334 noted that the so-called “iron loss pathway” (eq 43) reported by Wilkin and Barnes310 was actually a modification of the H2S reaction mechanism. The reaction stoichiometry proposed by Wilkin and Barnes (but not actually analytically demonstrated) was the sum of two wellestablished reactions:

FeS + H2S f FeS2p + H2

FeSm + 2H+ ) Fe2+ + H2S

(15)

FeSm + H2S ) FeS2p + H2

(51)

2FeSm + 2H+ ) FeS2p + Fe2+ + H2

(43)

7.4.5. Sulfide Controlled Reaction Kinetics: The Berzelius Reaction

(51)

where FeS was originally undefined but subsequently referred to as pyrrhotite337 or mackinawite.54 Reaction 51 is thermodynamically favorable at low temperatures (∆G°r ) -26.9 kJ mol-1, 25 °C, 1 bar total pressure where FeS ) FeSm) using the revised ∆G°f(FeSm) ) -98.22 kJ mol-1 and other data as listed above. Thus any discussions about its significance in the natural environment must solely be dependent on the kinetics. As discussed below, the kinetics of pyrite nucleation are characteristic of a sparingly soluble salt and quite sensitive to catalysis. It has become popular in some parts of the geochemical literature to sideline this reaction to higher temperature environments or late stage sedimentary processes, with little reason. In fact, as discussed below, the reaction is central to some of the most exciting developments in Fe-S chemistry at the present time. The Berzelius reaction is a special case of the Bunsen reaction where the Sn(-II) species has n ) 1 and the more stable form is protonated. It is important since S(-II) species are involved in all reported aqueous syntheses of pyrite. As noted above, the FMO sequence for Sn(-II) species suggests that HS- is less nucleophilic than Sn(-II) where n > 3. H2S is less nucleophilic than all the species listed since the HOMO for H2S is relatively stable (ca -10 eV).52 In contrast, the LUMO for H2S is around -1.1 eV,52 and therefore H2S is a good electron acceptor (i.e., oxidizing agent). The mechanism of the reaction between Fe(II) and H2S to form FeS2 thus again involves [FeS] as a reaction intermediate, but in this case, H2S does not substitute for [FeS]-S2- in a

Butler et al.334 noted that the reaction of FeSm (δ34S ) 2.8‰) and H2S (δ34S ) 3.3‰) by these processes would produce pyrite with an isotopic composition close to that of the reactant FeSm (and H2S), as observed by Wilkin and Barnes.310

7.4.6. Relative Rates of Pyrite Formation The rate laws for low-temperature pyrite formation in aqueous solutions have been established for pure inorganic systems. The major application of these laws is to elucidate the mechanisms of the pyrite-forming reactions and thereby inform various pyrite-forming processes, such as stable isotope fractionations or toxic element sequestration in sulfidic systems. The rate laws therefore provide a baseline for pyrite-formation rates in natural systems. As discussed above, the actual rates in natural systems can be affected by trace chemical catalysts and inhibitors273 and by organisms, especially microorganisms. The effect of microorganisms in the natural low-temperature sulfide system is to bring the systems closer to equilibrium. As discussed in section 2.1, for example, the inorganic sulfate-sulfide system is not reversible in low-temperature sulfidic environments because of the recalcitrant kinetics of inorganic sulfate reduction. Reversibility is achieved through the activities, for example, of sulfate-reducing prokaryotes. The effect of microorganisms


is therefore to increase the rates of reactions through catalytic effects of their biochemical pathways. This means that the experimentally measured rates of low-temperature inorganic systems may not describe the rates in any particular natural system. The rates derived from inorganic kinetic studies provide a sort of baseline rate, which may be locally enhanced depending, particularly, on the makeup the local microbial ecology. As noted above, some workers have limited the Berzelius reaction to higher temperature systems and have assumed that this reaction is far slower than the Bunsen reaction. In fact there is very little reason for this, and those reports do not include calculations of the relative rates under the conditions addressed. As shown above, in most pyriteforming experimental systems involving polysulfides and in all normal natural environments, the pyrite-forming environment includes both polysulfides and sulfides, as shown thermodynamically above. So both reactions occur in natural sulfidic environments.338 In the presence of FeSm, the rate is dependent specifically on the concentration of H2S on the one hand and polysulfides, especially the longer chain species with n ) 4 or 5, on the other. Therefore, the concentration of polysulfides or H2S will be rate-limiting. In systems undersaturated with respect to FeSm, the concentration of Fe(II) may become rate-limiting depending on the sulfide or polysulfide concentrations. We can ignore this situation initially in considering the relative rates of polysulfide and H2S reactions in sulfur-limited systems. The concentration of both polysulfide and H2S are pH dependent at similar total sulfide loadings. Thus the H2S concentration increases as the pH approaches 90%, this is equivalent to 1 and 100 µmol of FeSm/L of wet sediment. In this system, where [S(0)] ) 0 at 25 °C, and with k ) 10-4 mol L-1 s-1, the rate of pyrite formation is then between 10-11 and 10-6 µmol (L of wet sediment)-1 s-1 or 10-2-103 µmol of FeS2p (L of wet sediment)-1 a-1. Converting back to dry weight terms, this is equivalent to 3 × 10-4 to 3 × 101 µmol of FeS2p gdw-1 a-1. At pH ) 7, for the same conditions, [H2S] is between 0.5 µM and 0.5 mM, and the rate of pyrite formation 1.5 × 10-3 to 1.5 × 102 µmol FeS2p gdw-1 a-1. As the H2S concentration approaches 0 in alkaline sulfidic environments (e.g., pH > 8), the first term in eq 54 becomes very small, and the rate is dependent on S(0) and S(-II) concentrations and pH. As discussed above the [S(0)][S(II)] term in eq 54 actually describes the rate of formation of polysulfides. The application of eq 54 to sedimentary conditions is not straightforward since it depends on knowledge of both the concentration of S(0) and FeSm in the sediment and does not take into account the potential catalytic effects of microorganisms on the system. To illustrate the relative abiologic rates, we consider the case where the concentrations of S(0) and FeSm are of the same magnitude. The rate of pyrite formation for the polysulfide mechanism under the same environmental conditions as for the H2S mechanism, except that S(0) is present in equal molar quantities to FeS, then ranges between 10-2 and 10-11 µmol of FeS2p gdw-1 a-1. This is between 10-4 and 10-8 times slower than the H2S mechanism in the absence of S(0). Even accepting considerable errors in the estimate of FeSm and S(0) concentrations, the differences are so great that the polysulfide mechanism is relatively slow except under conditions where S(0) is present in significant concentrations. Thus, although the rate of pyrite formation may be described mainly by the kinetics of the Bunsen reaction initially near the redox cline, pyrite is actually formed relatively slowly by this process, at least in abiologic systems. However, the microbial ecology of the anoxic/oxic boundary in sulfidic environments includes sulfur-disproportionating microorganisms, which are closely involved in sedimentary pyrite formation in this part of the system. Isotopic evidence338 shows that pyrite forms at similar rates through both the polysulfide and H2S processes during bacterial sulfur disproportionation, with total rates up to 105 times faster than the purely inorganic process would suggest. The nature of the catalytic processes enabled by these organisms is unknown. However, it is associated with extremely large sulfur isotope fractionations, and the bacteria leave the pyrite with this signature. The effect of microorganisms on the kinetics is a reflection of their effect


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on the thermodynamics of aqueous sulfide systems discussed above. Microorganisms bring low-temperature sulfide systems closer to equilibrium. In other words, they increase the reaction rates. In iron-limited systems, the rate is controlled by the concentration of Fe(II). In eq 54, this is expressed as [FeS]. Obviously as the Fe(II) concentration approached zero, eq 54 shows that the rate of pyrite formation tends to zero. The effect of Fe limitation is greater on the polysulfide reaction since the second term in eq 54 shows a second-order dependence on [FeS]. As Fe(II) becomes limiting therefore, in the presence of equal concentrations of H2S and Sn(-II), the sulfide reaction will dominate the pyrite formation rate.

7.4.7. Kinetics of Pyrite Crystal Growth Pyrite “formation” or “precipitation” consists of two processes: (1) nucleation, which is slow and requires relatively high supersaturations, catalytic effects of active surfaces, or both or added trace components, and (2) crystal growth, which is relatively rapid and occurs through the reaction between Fe2+ and S2(-II). The equilibrium data discussed above shows that the disulfide species HS2- is present in sufficient concentrations in any experimental or natural S(-II) solution to account for pyrite formation. The experimental systems were saturated with respect to FeSm, and the resultant FeS2p was produced by reaction between FeS0 and sulfur species. This is demonstrated by the observations that (a) the mechanism involves a solution reaction and (b) the suppression of FeS0 led to the inhibition of FeS2p formation.273 Homogeneous nucleation in solution is unexpected at the best of times, and homogeneous nucleation of relatively insoluble salts like FeS2p is unlikely. Harmandas et al.306 showed that the pyrite surface itself could act as a catalyst to allow rapid nucleation of FeS2p such that the rate-controlling reaction was crystal growth. In order to achieve this relatively rapidly, Ω needs to be in excess of 1014 at 25 °C and pH ) 6.5. As discussed above, experimental studies of pyrite formation have been limited by technical problems in probing the system at dissolved Fe and S values that would be below saturation with FeSm. However, Harmandas et al.306 achieved this, and produced measurable amounts of FeS2p, using a constant pH system and pyrite seed crystals. The reactants were hexaqua Fe2+, added in the form of Fe(NH4)2(SO4)2, and S(-II) added as NaHS. The pH was maintained at 6.5 and the temperature at 25 °C. The relative saturation with respect to pyrite was calculated according to the reaction

Fe2+ + S22- ) FeS2p

(55)

It is unclear how {S22-} was calculated. Polysulfide stability constants were not listed in their Table 1, and they merely note (p 1252), “Computations of the relative supersaturation were done considering within the equilibria the presence of disulfide ions.” They measured the rate of pyrite crystal growth, Rp, and showed that it fitted the semiempirical equation

Rp ) ksΩpn

(56)

where n ) 3.5 ( 0.5, k is the growth rate constant, and s is a function of the active growth sites on the crystal seeds. The rate of pyrite crystal growth is given in units of mol

m-2 s-1, which varied from 2.3 × 10-8 to 2.1 × 10-7 at Ωp between 1.1 × 107 and 2.4 × 107. The rate law and high value for n suggest a surface diffusion controlled growth mechanism, which is expected for sparingly soluble salts. Harmandas et al. reported that a similar order was found for the spontaneous precipitation of FeS2p at 80 °C. It is therefore quite clear that pyrite crystals grow quite rapidly from a reaction involving Fe2+ and S2(-II) and that there is no requirement for a FeSm intermediate in the reaction. Pyrite has also been synthesized from undersaturation with respect to FeSm or FeS0 within plant cells.329,339 In these experiments, initially precipitated FeSm in the open outer xylem cells was shown to dissolve, and FeS2p formed in the closed interior parenchyma cells. As described above, calculations based on these experiments329 show that organic surfaces provide an enhanced catalytic effect compared with a pyrite surface, with lower Ω values being needed to produce pyrite rapidly. It therefore appears that pyrite can nucleate from the reaction between hexaqua Fe2+ and S2(-II) in the presence of an active surface. It is possible that other iron sulfides, such as FeSm and Fe3S4g provide a similar role. Certainly the intimate association observed between FeS2p and Fe3S4g in some natural settings would support this proposition.

8. Other Iron Sulfides We have considered the chemistry of the main iron sulfides in marine systems above. However, as shown in Table 1, a number of other iron sulfides occur in nature and have been synthesized in the laboratory. In the following, we briefly consider these phases.

8.1. Cubic FeSc Cubic FeSc is the Fe equivalent of sphalerite, cubic ZnS. It mainly forms as a product of the reaction of H2S with metallic Fe. It was first identified by de Me´dicis340,341 and Takeno et al.342 It is a corrosion product in the Girdler sulfide process of heavy-water extraction.343 FeSc is formed at 92 °C, troilite (FeSt) and FeSm form. At pH > 6, FeSm forms. FeSc is metastable with respect to FeSm and FeSt and converts to these phases at room temperature in hours to days. Murowchick and Barnes344 note that the reactants and products detected in the reaction suggest that FeSc forms through competitive reactions between metallic Fe and S(-II) and aqueous Fe2+ and S(-II) with the evolution of H2 gas. The reason for the formation of this phase is not understood. The sphalerite structure of FeSc includes Fe4S62- subunits and thus [Fe4S6(H2O)4]2- clusters may be necessary solution precursors by analogy with the ZnS system.345 As discussed above, the common form of aqueous FeS clusters is FenSn, which are directly related to the basic structural moiety of FeSm. The formation of FeSc may not therefore be enabled in most FeS solutions. The limitation of FeSc formation to lower pH may be related to the changing nature of the aqueous FeS clusters in more acid solutions as discussed above. Interestingly, one recipe for forming large, micrometer-sized (everything is relative), FeSm crystals is through the electrochemical reaction between Fe(0) and S(-II). It may be that the presence of a FeSc template permits these larger FeSm crystals to form in contrast to the nanoparticulate material produced through the direct solution reaction. FeSc has not been found in nature and present



evidence, including its short lifespan and relationship with metallic Fe, makes this unlikely.

8.2. Troilite, FeSt, and the Pyrrhotite Group Pyrrhotite has only been reported once in sediments and troilite has not been reported.1 The pyrrhotite occurrence was as an overgrowth on magnetite from the Sea of Japan.346 This association may suggest a higher temperature source. Neal et al.347 reported monoclinic pyrrhotite formed on hematite surfaces after 6 months exposure to sulfate-reducing bacteria although this phase has not been identified in similar experimental studies with these organisms.101,270,348 Likewise, Farina et al.349 described the formation of pyrrhotite in intracellular granules of magnetotactic bacteria, but Posfai et al.256 found no evidence for pyrrhotite in these organisms. So the jury is out. There is no doubt that pyrrhotites have been observed experimentally in low-temperature aqueous systems with and without various types of bacteria. However, the process of formation is not understood and does not seem to be readily reproducible. Of course, the consequence of pyrrhotites being formed in these systems would be that these minerals might yet be formed more often in marine systems. The pyrrhotite group includes a number of FeS phases based on the NiAs structure displaying various superstructures caused by ordered vacancy geometries in the nonstoichiometric composition. Two basic subgroups are observed: the hexagonal pyrrhotites, which tend to be ironrich with compositions centering around Fe10S11, and the monoclinic pyrrhotites, which tend to be less iron-rich with compositions generally around Fe7S8. Monoclinic pyrrhotites tend to be stable relative to the hexagonal forms at temperatures less than around 200 °C. Troilite, FeSt, is the stoichometric end-member phase. Although Fe1-xS and FeSt are stable relative to FeSm, they are rarely observed in marine systems. The reason is shown in the pH-Eh diagram (Figure 35). In low-temperature aqueous solutions, pyrite is the stable phase. As discussed above, the relative stability of the HS22- species in sulfidic solutions combined with the low solubility of pyrite makes this phase unavoidable in terms of equilibrium thermodynamics. If FeSt and Fe1-xS are to be produced in these systems, some kinetic factors, such as the inhibition of pyrite nucleation and the catalysis of FeSt and Fe1-xS formation, must be involved. We noted above, for example, that FeSt may be produced through the reaction between acidic aqueous S(-II) solutions and Fe(0) either directly >92 °C or through a FeSc intermediary. However, the predominance of pyrite in aqueous equilibrium systems does not explain why FeSm, in particular, forms in place of FeSt or Fe1-xS. The explanation for this is, as discussed above, the identity of the FeSm structural moiety with the Fe2S2-type aqueous clusters. The topology of the FeSt structure is complex with both Fe2S2 and Fe3S3 rings.350 Taylor351 noted that a trimeric precursor Fe3S3 may be required for the nucleation of FeSt. Fe3S3 rings have not been identified in aqueous FeS solutions although their occurrence is not precluded. A kinetic explanation may be consistent with the conflicting reports of pyrrhotite syntheses in low-temperature aqueous solutions reported above.

8.3. Smythite, Rhombohedral Fe9S11s 352

Smythite, rhombohedral Fe9S11s, was originally thought to be a greigite polymorph, Fe3S4s.353 The revised formulation is equivalent to Fe3.27S4. However, Fleet354 suggested on

structural grounds that the ideal formula was Fe13S16, which would bring it close to Fe3.25S4. The problem is that the natural materials on which the composition is based normally include various amounts of Ni and, as discussed above with respect to FeSm and FeSp, this lends a degree of uncertainty to the composition. It was originally synthesized by Rickard355 and is a pure member of the Fe-S system. The synthesis was confirmed some 28 years later by Furukawa and Barnes.356 Rickard noted the coincidence between the rhombohedral structure of Fe9S11s and that of siderite, FeCO3, and suggested that the material was formed by an epitactic reaction. In nature, Fe9S11s is usually found associated with rhombohedral carbonates. However, it is also found in pyrrhotites, where it is generally Ni-rich and appears to be produced through exolution on rapid quenching.354,356 Furukawa and Barnes356 found that the upper temperature limit of Fe9S11s formation through the reaction with siderite was 53 °C and the pH needed to be >6. These conditions appear to be possible in some marine, or at least estuarine, environments, but Fe9S11s has not yet been reported from these systems.

8.4. Marcasite, Orthorhombic FeS2m Marcasite is the orthorhombic dimorph of pyrite. Its formation remains somewhat of a mystery. Syntheses of FeS2m require pH < 6, and the rate of formation becomes rapid at pH < 4.357 It has been suggested that the presence of protonated Sn(-II) species, which may dominate in acid solutions as discussed above, is somehow related to its formation.358 This mirrors an older idea that FeS2m includes H in its structure, but this is not the case. A kinetic study of FeS2m formation would probably help solve the problem. Marcasite is not found in normal marine sediments, although it is a common constituent of deep ocean hydrothermal vent mineralization. The reason for the lack of marine FeS2m is probably the pH requirements for its formation. FeS2m is a common constituent of ancient marine sediments, however, where it is produced by circulating acidic groundwaters.

8.5. Other Iron Sulfides For completeness, it should be mentioned that a number of other iron sulfides have been reported in the literature. The case of Fe2S3 has been mentioned above. However, it continues to be reported, most recently as an hexagonal phase within synthetic pyrrhotites.359 A variety of FeSm has been reported as a product of a synthetic reaction between ferrihydrite, goethite, or hematite and S(-II) with a d-spacing exactly double that of mackinawite.360 The material is only produced in high NaCl concentrations and its composition appears variable. It is reported to change to FeSm with time. In this context, it is also worth noting that a variety of alkali iron sulfides are known and have been reported as minerals from highly saline environments. These materials are easily synthesized in low-temperature aqueous systems, have definitive compositions, and are remarkably stable compared with the usual FeS precipitates. It is possible that they may have or have had some role in extreme marine systems in very alkaline microenvironments.

9. The FeS System in Biochemistry and for Organic Compound Formation Iron-sulfur clusters are agents that affect many biochemical processes as they are the active sites in proteins with


molecular masses starting from about 6000 Da. They are common to most ancient components of living matter and are present in a host of other organisms such as photosynthetic organisms, nitrogen-fixing bacteria, and submitochondrial fractions of mammalian origin.361 Recently in this journal, Holm and co-workers362 have reviewed synthetic analogues of iron-sulfur clusters. The main cluster stoichiometries found are Fe2S2(SR)2, Fe3S4(SR)3, Fe4S4(SR)4, and dimers of the latter. In some enzymes containing Fe4S4(SR)4 and higher stoichiometries, one or more of the Fe atoms is replaced by another metal (e.g., Ni, Mo, or W). The Fe2S2 structure has a core similar to that found in mackinawite whereas the other structures are cubane-like (Figure 19). Most syntheses of FeS clusters are performed in nonaqueous solvents to prevent iron sulfide precipitation. One of the most interesting examples is the formation of the inorganic cluster [Fe4S4(SH)4]2- in nonaqueous solvents193,363,364 (Figure 19). This is the simplest synthetic analog of a ferredoxin as SH replaces thiol groups (SR). The ferredoxin clusters containing thiols form readily in aqueous solution193,365 and are examples of self-assembly reactions. Thus they are considered possible starting materials or catalysts for organic compound formation and the origins of life. In the last 20 years, there has been much interest in the Fe/S system as a strong reducing system, as a catalytic system in the formation of organic compounds necessary for life, and as a geochemical driver for biochemical processes. The hypotheses are grouped in the “iron-sulfur world” concept, an idea that iron sulfides played an extensive role in prebiotic organic syntheses on the early Earth. Two groups have provided detailed theories for the evolution of organic compounds necessary for life using iron-sulfur chemistry. Wa¨chtershaüser and co-workers366-371 proposed the hydrothermal synthesis of FeS2, which requires acidic conditions, whereas Russell and co-workers372-375 proposed the hydrothermal synthesis of FeS species but at lower temperatures and under more alkaline conditions. Both processes have FeS species as important components and reactants. Key in both of these hypotheses is how CO2 can be reduced to form organic compounds. A major stumbling block is that the LUMO orbital of CO2 is positive in potential energy (+0.60 eV376) and CO2 has a very negative reduction potential (-2.22 V vs SCE377). These data indicate that CO2 is not an efficient electron acceptor and requires very strong reducing conditions for its reduction.

9.1. Pyrite Formation Proposal Wa¨chtershaüser287,366 proposed the hydrothermal synthesis of organic compounds during the inorganic synthesis of FeS2 as a primary pathway. The initial step was the reduction of CO2 to small organic molecules, particularly thiolated methanoic (formic) acid, HSCH2OOH. In the assumed acidic early Earth ocean,378 sulfur as H2S would have dominated sulfur speciation,379 so pyrite formation through the reaction between H2S and FeS (eq 51) was proposed as a new energy source for the autotrophic origin of life because of the H2 produced. H2 could then be used directly by organisms for growth or for the reduction of CO2, organic compounds, and nitrate.370 Drobner et al.369 documented the reaction, and Rickard54 determined the rate law from 25 to 125 °C with FeSm as the reactant (eq 51). As noted above, FeS was nonprescriptive, and pyrrhotites were involved in the original hypothesis. However, Schoonen et al.294 found that direct

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electron transfer from the valence band of pyrrhotite to the lowest unoccupied molecular orbital of CO2 would not occur. Since the original proposal, the reaction with FeS and H2S alone or with NiS present has been shown to produce organic sulfur compounds, but not formate, when CO2 is added to the reaction mixture.370,371,380,381 In this case, CO2 is converted to OCS and CS2 [by FeS and H2S], which are then further reduced by H2 to form simple organic sulfur compounds such as CH3SH and CH3SSCH3, as well as amide bonds when reduced nitrogen is present. CH3SH is an excellent starting material because CO insertion forms CH3(CO)SH, which is a precursor for acetyl-CoA in the reverse or reductive citrate cycle.337 Cody382 reviewed metal sulfides and detailed their potential importance in the origins of metabolism. The mechanisms for the above reactions are presently unknown but appear to be related to addition of sulfide to CO2 to form the monothiocarbonate ion, HCSO2-, which can decompose to OCS.383 The reducing power of the FeS/ FeS2 couple to reduce CO2 to formate and simple amino acids was questioned as an important process by Keefe et al.384 based on experimental data and by Schoonen et al.294 based on thermodynamic calculations. The latter group demonstrated that with increasing temperature eq 51 has less reducing power and above 250 °C its reducing power is less than that of other mineral assemblages such as quartzfayalite-magnetite and pyrrhotite-pyrite-magnetite. Thus CO2 reduction to formate is less likely by the FeS2 formation reaction. At the time of the Schoonen et al. study, the thermodynamic argument did not address three relevant points that may permit reduction of CO2 to formate via FeS2 formation. First, large black smokers (Figure 38A) at hydrothermal vents with temperatures of 360 °C are not the only source of FeS and H2S. In the absence of substantial sulfate in the ancient Earth oceans,379 it is improbable that large sulfide edifices such as those in the present-day deep oceans could be produced. Even today, diffuse flow hydrothermal fluids,385-387 which are typically less than 100 °C, release more material volumetrically than that from black smokers at vents. Diffuse flow fluids also contain FeS and H2S.200 In addition to diffuse flow systems, small chimneys and beehive diffusers388 are porous structures (Figures 38B and 39) associated with large black smoker edifices that are formed at significantly lower temperatures than black smokers. Beehive diffusers have been identified as ideal natural reaction kettles for potential prebiotic organic syntheses.373 One problem with the iron-sulfur world hypothesis for the origin of life is that although all reactions from CO to peptides, for example, have been reproduced in the laboratory,389 these syntheses were achieved under a wide range of conditions, especially temperature. In this context, beehive diffusers appear to be interesting natural reactors. However, the modern versions are anhydrite cemented388 and thus unlikely to be formed in a sulfate-limited Hadean ocean. Also, they mix fluids over a steep thermal gradient from hydrothermal temperatures in the interior to deep ocean temperatures of 2 °C at the surface. As suggested by the analyzed structure in Figure 39, the conditions may vary from 3 to 360 °C, from oxidized to reduced, and from acid to alkaline over short distances. The modern day organic chemistry of beehive diffusers has not been investigated and could be an interesting target for research. If similar natural reactors were present in the prebiotic Earth oceans, they



Figure 39. Elements of a beehive diffuser from an edifice at Broken Spur (29° 10′ N) from Rickard et al.388 Reproduced with permission of the Mineralogical Society of Great Britain & Ireland.

Figure 38. (A) Black smoker hydrothermal vent probed by an electrode wand 0.5 m above the vent chimney (e360 °C) at a depth of 2.5 km on the North East Pacific Rise (9° 50′ N), (B) beehive structure from the North East Pacific Rise (9° 50′ N), and (C) Reproducible cyclic voltammetry scans (1 V/s) taken 0.5 m above the vent orifice (location is the wand tip in Figure 38A) where the temperature was 25 °C and the pH ≈ 6. Only H2S and FeS(aq) were detected. The total sulfide (SAVS) was recovered on the negative scan.

would necessarily have been different from the modern-day examples. Second, Schoonen et al.294 assumed that FeS was a solid entity, but Luther and Rickard166 and Theberge and Luther173 have shown that there is a soluble form of FeS [FeS(aq)],

which has properties similar to molecular clusters found in ferrodoxins that may enhance the reaction. The Fe in [FeS(aq)] has facile ligand exchange and the S in [FeS(aq)] has electron-donating capability that permits reaction at the C atom in CO2 to activate CO2. A similar process has been shown in biotin formation from ferredoxin Fe4S4 clusters.390 The pH at which all these reactions are performed is probably very critical since the H2S/HS- system can interact with CO2 to produce OCS and CS2 on the way to organic compounds.373 Third, electrochemical reduction of CO2 to formate on pyrite has now been documented.391 CO2 reduction is not the only pathway for organic compound formation using the Fe-S system. CO insertion into simple organic molecules has been investigated in subsequent work. Cody et al.392 suggested the Fe-S system (with CO molecules possibly attached to Fe) as a prime catalyst for the organic synthesis of pyruvate under extreme temperature and pressure. In that work the catalyst was speculated to be Fe2(RS)2(CO)6 because of spectroscopic evidence and because it can be converted to Fe2(RS)4S22-. Also, peptides were formed with (Ni,Fe)S surfaces with CO and amino acids, but the structure of the catalyst was not identified.393 Trapping the catalyst is not an easy task, but spectroscopic techniques can give a clue to the catalyst’s identity.390,392 Clearly more work is needed in this area to better understand reactivity and possible mechanisms. The problem centers on the same dichotomy between equilibrium


thermodynamics and kinetics that has been a continuous thread throughout this review. Equilibrium thermodynamics not only describes the state of the system but is also constrained by the choice of reactants and products. Thus, calculations such as those of Schoonen et al.294 are unquestionably correct; however, a different result might be obtained with soluble FeS catalysts or intermediates as reactants.

9.2. FeS Membrane Proposal The other proposed use of the FeS system comes from Russell and colleagues.372-375 In this proposal, life emerged from a redox and pH front about 4.2 Ga ago (Ga ) gigaannum or billion years). Here hot alkaline bisulfide-bearing submarine seep waters emerged into the Hadean ocean, which was acidic (due to high CO2 levels), warm, and ironbearing. At the interface of these two aquatic systems, an FeS colloidal membrane spontaneously formed. This semipermeable membrane contained Ni and acted as a catalytic boundary for the transfer of electrons for the reduction of CO2 to organic acids and anions as well as the hydrogenation and CO/CO2 insertion of other hydrothermal organic primers. Russell views the chemistry of the FeS membrane as a ferredoxin-like material, which may have been the precursor to the use of ferredoxins in living organisms. One of the problems with this idea is that FeSm does not form large crystals but is commonly nanoparticulate. Filtness et al.394 synthesized a pure FeSm “membrane” and found that it behaved more like a permeable reactive barrier than a membrane. Still the Russell proposal is attractive for several reasons. First, the reaction of CO2 with FeS alone or with H2S has resulted in OCS, CS2, and organic sulfur compound formation including CH3SH380 indicating that FeS is a strong reductant. The FeS HOMO (∼0.5 eV and near in energy to the LUMO of CO2) is less stable than the FeS2 HOMO.51,291,294,383 Thus, the HOMO of FeS is an excellent electron-donor orbital and should interact better with all the LUMO orbitals of CO2, OCS, and CS2 in a manner similar to that of Zn-OH in carbonic anhydrase.395 Second, solid and soluble FeS clusters have high-spin Fe atoms in tetrahedral coordination, and electron transfer is facilitated with this configuration over low-spin and octahedral Fe in FeS2. The high-spin and tetrahedral Fe in FeS clusters permits other metals such as Ni, W, and Mo to replace an Fe atom in the cluster. Known biochemical FeS clusters have similar structural features as those found in mackinawite, greigite, and NiFeS minerals,373,382 so FeS bubble formation likely resulted in FeS clusters and efficient electron transfer as well. The replacement of a Ni atom for an Fe atom in an FeS cluster396 has been shown to enhance organic compound formation as has the use of mixed FeS/NiS solid phases.381,393 Last, the electroreduction of CO2 is catalyzed by Fe4S4 clusters397 and can shift the electrode reduction potential for CO2 by about + 0.7 V in nonaqueous solvents. Although Russell’s hypothesis was for the Hadean ocean, other interfaces including hydrothermal vents and sediments that show profiles as in Figure 2 are likely places for present day occurrence of this pathway. The beehive structure in Figure 38 is an example of a present day environment that is porous and that fits the requirements of the Russell hypothesis. These two Fe-S hypotheses indicate that hydrothermal vents, hot springs, sediments, and the sub-seafloor biosphere are ideal chemical sources for FeS clusters, nanoparticles,

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and solids. Also, these extreme environments are possible systems for the evolution of organic molecules and life since they provide not only Fe-S chemistry but many of the other transition metals that are essential to life in micromolar to millimolar concentrations.398 There is some exciting chemistry out there.

10. Acknowledgments D.R. acknowledges NERC Grant NRE/L/S/2000/00611. G.W.L. acknowledges support from NSF Grants OCE0240896 and OCE-0326434. We thank Ovadia Lev for comments on an earlier version of this paper. Reviewing this paper was not a trivial exercise: We thank John Morse and two anonymous referees for their heroic efforts, which did much to improve the manuscript. We thank Miha´ly Posfai for providing a high-resolution version of his electron micrograph, originally published in Science and reproduced in Figure 32.

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